I feel that Focus Sash should be the first item slashed in, followed by LO and then Rocky Helmet. Rocky Helmet is pointless on a Lead set, as Rough Skin already ensures that physical attackers lose their own sash or take some damage in return for their attacks. Many pokemon lead with Ice attacks including but not limited to Mamoswine, Azelf, and Tyranitar. Scarfed dragons and any Lati@s also manhandle Garchomp without a sash, and it really isn't the type of pokemon you'd want going down in turn one after (or worse, before) it sets up SR. Far too many pokemon outspeed and KO Garchomp with Hidden Power Ice, so I really think that it's not a very solid option to be using. Rocky Helmet helps Garchomp spinblock when spinners are at terribly low percentages, but Garchomp already KOs Forry with Fire blast and Tentacruel with EQ. Starmie isn't going to care about Rocky Helmet, as it can simply outspeed and KO Garchomp with Ice beam. The only spinner Rocky Helmet might help with is Donphan, as it can't be KOed too easily by Garchomp, but as a lead set, with the comment "Because this variant of Garchomp isn't necessarily designed to stick around the entire match," Rocky Helmet isn't a very solid option.
I honestly don't mean to knock on Rocky Helmet Garchomp, as it is an incredible support pokemon with tremendous offensive presence. However, I think that the SR set changes completely when putting a Rocky Helmet on it, as the Garchomp should then invest in more physical and defensive bulk, and function in a similar manner to the classic Support Garchomp set that is currently up. It's kind of awkward, with two very sets competing for the same Stealth Rock name. The Focus Sash set, which functions more like a Suicide Lead capable of weakening defensive walls, roasting Steels, and punching through slower teams does benefit from an attack boosting nature as a lead, as stuff that already outspeeds Jollychomp will still outspeed Adamant Garchomp, while other leads are for the most part, much slower. More offensive Garchomp sets in the lead slot tend to be placed on HO teams, while Rocky Helmet might on a bulkier, more balanced help punish U-turn users and spinners. I'd imagine that a Garchomp running Rocky Helmet is going to be around for longer to potentially set up Stealth Rocks multiple times while helping to punish U-turn and deter weaker physical attacks by threatening a switch and Rocky Helmet/Rough Skin damage.
The SD set is very potent against bulkier, slower teams, but against faster teams, this Garchomp usually finds itself unable to KO more than one pokemon if its lucky, before being forced out by anything threatening HP Ice, Ice Beam, or a Scarfed Dragon attack. I agree with you and many posters in the thread that Yache is the best item to use on this set, but I can't help but feel Garchomp is still a little too slow sitting around its 102 speed tier and might enjoy having a Substitute up in some cases.
For this reason, I think that the Scarf set is one of Garchomp's premiere sets in OU (at least better than SD) as it takes Garchomps above-average speed and gives it the Jump on every non-scarfed pokemon. Many people use a scarfer on their team to outspeed and KO faster pokemon, but also to handle +1/+1 Dragonite, +1/+1/+1 Volcarona, and Moxiemence which threaten to sweep their respective boosts. Very few scarfers are capable of KOing both without a boost, and Garchomp handles both very reliably. With a Scarf, Garchomp can destroy every single pokemon in OU that outspeeds it (except for Keldeo, who takes ~72%) while checking the most dangerous boosting pokemon in OU. I think it should be mentioned that Dual Chop should always be run over Dragon Claw, as this Garchomp can outspeed and KO its nemesis, SashZam, with Dual Chop 87.5% of the time (Dugtrio too of course). I suppose it is fairly predictable, and sucks when locked into Outrage, but Choice Scarf was the #1 used item on Garchomp for a reason: it's a very good set.
I feel that the Banded set is terribly outclassed by Multiscale Dragonite (Garchomp has STAB on EQ, but it's unneeded power, as CB nonSTAB EQ from Dnite lready KOs 252/108 Jirachi, and Garchomp still isn't 2HKOing standard Ferrothorn with it; CB Aqua Tail is very underrated, but Dragonite gets the same move and functions better on Rain teams), and is one of Garchomp's worst sets =/
To summarize: The SR set is very confusing as it's got too many slashes and spreads, without too much comment on Rocky Helmet, a good item but not a good item for Lead Garchomp. There exists the variant that gives up a coverage move for Stealth rocks, but continues to function as an ordinary Garchomp. There is another variant which functions like lead Ttar or other Suicide Leads which threatens to KO many opposing Leads while guaranteeing Stealth Rocks before dying. SD allows it to function kind of like tauntSR terrakion, being a powerful, physical threat that sets up rocks or uses SD to start punching some holes in the opposing team. Rocky helmet has no place on a HO lead of this sort. In fact, the line "Rocky Helmet allows Garchomp to compound residual damage against Physical attackers, such as Scizor or Terrakion," makes no sense as Garchomp can roast Scizor with Fire Blast to begin with, while KOing Terrakion with a combination of EQ and Rough Skin damage, getting up Stealth rocks as Terrakion CCs (if it used rocks turn 1) or straight up KOing Terrakion without it getting up any rocks in the first turn of the battle. Rocky Helmet would probably function best with the bulkier spread listed in the comments below. I also think that lead Garchomp can benefit potentially from split offenses to give it a solid shot at KOing Skarmory/Ferrothorn with an Attack Boosting nature, giving it actually more physical and special power than the typical Jolly/Hasty spread. Comments like "X nature if using Fire Blast and Y nature if using SD" would probably also help to guide newer users.AGH the summary got rather lengthy
tldr; SR set is very confusing with Rocky helmet used. LO is by far the superior option on a HO suicide lead set. For something with more bulk or meant to last longer in the match to potentially get up rocks again or just deter physical attackers, the bulky spread with Rocky Helmet is amazing. Further, it would be helpful if the scarf set mentions that Dual chop almost always lets chomp KO zam, and even better if its moved about the SD set. Just because something works in WCop doesn't mean it's solid on OU ladder play, as surprise value sets gain much more potential in single WCop matches, but don't necessarily guarantee solid, consistent success.
I'm extremely sorry if I need a badge to post in this thread, but I haven't been able to find any Rules or Guidelines for revamp threads like these, and would love to help, particularly because I feel Garchomp's Scarfset is criminally underrated, and also because I'd love to help in any way possible. Please delete my comment if I've made a mistake or if this post isn't useful.