Yeah, not the Hyper Beams; the real boons of Slowbro's physical bulk are in taking Crunch/Thunder Fang from Aerodactyle and STAB Outrage.
I don't like the idea of relying on Nasty Plot as a crutch, and then saying "hey, it's okay to not have Slack Off because you can Hyper Potion," because that's ultimately the same result as throwing on X-items onto Pokemon to boost their stats. If X-Acc + Horn Drill doesn't jump a Pokemon to Top Tier in RBY's tier list, Nast Plot + H-Potion shouldn't push a Poke to the top of this one either.
I don't know what games you've been playing, but 5 levels means a lot in a Pokemon's performance-- In game, you're trying to avoid grinding for every level, and you're trying to avoid wasting trainer battles (experience) on Pokemon. If you can get them at a higher level instead, that's a big deal. Especially in HGSS where there is so little available experience from trainer battles, and your team is liable to still be in its late 30's going into the Elite Four.
This carries on to Slowpoke as well. Why would I use a severely under-leveled Pokemon that will be a minor contributor at best? When the alternative is to get a high-leveled fully evolved Poke right at the point in the game where it will start to excel?
Even if you can raise it to beat Metapod/Kakuna, who cares? It can Yawn Miltank-- useful, but not a game breaker. Similarly, Confusion-ing a bunch of Ghastlies to death isn't a big achievement either. I could use those trainer battles to put valuable experience on Pokemon who can really perform against those gyms-- Pokemon who are best trained in this earlier arc of the game. It makes no sense to waste experience on a Slowpoke who will probably end up at only around level 24-27 after Ekruteak when I could catch a level 30 Slowbro at the same part of the game.
I'll add some notes about Physical Bro later though, and mention Yawn and Curse. I wouldn't use a full physical Bro in-game because Cursing every battle is pretty inefficient, and Surf is powerful and convenient, while Aqua Tail / Dive are pretty damn inconvenient. Avalanche is cool though-- it appears right before you really need Ice moves, and it's much more reliable than Blizzard. Avalanche would be the main reason you'd go for a Cursing/Physical Slowbro. I'll be sure to add these details in.
"Hmmm... a name for this plan..."