This thread is really helpful. While looking through, I became a little confused about the way some Pokemon are determined as GSI/SSI/NSI, but here are a few thoughts I had:
Mega Ampharos as a GSI for Mega Scizor seems shaky. Offensive M-Sciz is faster and does 90 - 106% with +2 Bug Bite to SpD M-Amphy, while KOing Agility variants. With the analysis spread for bulky SD M-Sciz, Agility M-Amphy loses coming in on SD, since Bug Bite/Superpower followed by BP kills. In the process, M-Sciz takes 46.6 - 55.1% from a Modest Thunderbolt, which can be healed later. SpD M-Amphy is also set up fodder for bulky M-Sciz. The only scenario I can see M-Amphy switching in and winning is when coming in on a bulky SD variants with Knock Off, which I don't think is as common as other coverage options. In this situation, Agility M-Amphy will be faster and have a good chance to 2HKO with Thunderbolt if using Modest, while surviving any combination of hits. However, I wouldn't say that this makes M-Amphy a GSI, so maybe that could be edited if others agree.
On the topic of M-Scizor, is there a reason it's listed as a SSI to Mamoswine, while regular Scizor is listed as a GSI to it? M-Scizor should be a GSI if regular Scizor is too. Its superior bulk lets it take LO EQ much better than its regular form. It can be invested heavily in physical defense to avoid a 2HKO from even Adamant LO EQ, and Roost off the damage. Regular Sciz can't do this as comfortably, and is hit for a 2HKO even with max defense and Impish, unlike its mega form. The only argument I can think of for regular Sciz being a GSI over its mega form is being able to switch in on EQ with its CB set and KO back with BP, whereas max attack M-Sciz can't do the same and KO in return. However, that seems like a weird reason, and don't think it justifies M-Sciz only being an SSI.
Rotom-W as a GSI to Bisharp is a little misleading. I know it can come in on a predicted Sucker or Iron Head, but if it switches in on SD, it's dead to Knock Off. Speedy Rotom-W with Bold can take a Knock Off and proceed to burn and win between Pain Split + Hydro Pump, but those variants aren't very common and some Bisharp use Lum Berry, making this situational at best. As such, I think Rotom-W would fit better under the SSI category in this case.
Hippowdon as a GSI for Mega Houndoom is a bit questionable, no? Fire Blast has a 91.8% chance to 2HKO the standard mixed spread, doing 51.4 - 60.7%. If doggy NPs on the switch, that's a clean kill. SpD Hippo switches in more reliably, but these variants aren't too common and is unlisted as a main set on the analysis. Even with a SpD spread, though, you can argue that Mega Houndoom can burn Hippo on the switch, NP on the EQ, and win. While very uncommon, there's also Sunny Day sets capable of blasting through even SpD Hippo with a Solar Power boosted FB. I think SSI would be more accurate here.
I am slightly unsure about this one, but regular Gyarados as a GSI to Manaphy seems arguable. RestTalk variants can come in and phaze, but is that doing anything other than delaying Manaphy's second attempt at sweeping? On the initial switch, you will have to take a boosted Energy Ball. If you factor in SR, Gyara will be taking 60+ damage. If you can't find the opportunity to Rest before Manaphy comes in again, you are likely dying. If you do find the chance to Rest before Manaphy comes back in, you have to rely on good Sleep Talk rolls to phaze, which is unreliable. In 1v1 situations, RestTalk Gyara can't beat any variant of Manaphy either because it doesn't do enough damage with its attacks. Further, the more common DD variants can't switch in on Tail Glow sets and win, because Manaphy takes +1 Bounce and 2HKOs back. Gyara also has to be wary of Scald burns. I can understand the reasoning behind Gyara in GSI, but it's a poor answer overall.
There's some other placements I found odd, but am hesitant to mention until I think about them more :s
