I did all of my testing on USUM version 1.2.
First round of tests: Staraptor 1 has 150 Speed, Staraptor 2 has 151 Speed
1) Choice Scarf (does not affect)
Staraptor 1 holds Choice Scarf with 150 Speed, Final Gambits Staraptor 2 with 151 Speed. Replacement was in order of Staraptor 2, then Staraptor 1.
2) Stat boosts (does not affect)
Staraptor 1 is at +2 Speed from Agility with 150 Speed, Final Gambits Staraptor 2 with 151 Speed. Replacement was in order of Staraptor 2, then Staraptor 1.
3) Tailwind (affects)
- Staraptor 1 is under the effects of Tailwind with 150 Speed, Final Gambits Staraptor 2 with 151 Speed. Replacement was in order of Staraptor 1, then Staraptor 2.
- Also tested: same situation, but Tailwind expires on this turn before the next Pokémon is replaced. Replacement was in order of Staraptor 2, then Staraptor 1.
Second round of tests: Staraptor 1 has 151 Speed, Staraptor 2 has 150 Speed
4) Iron Ball (does not affect)
Staraptor 1 holds Iron Ball with 151 Speed, Final Gambits Staraptor 2 with 150 Speed. Replacement was in order of Staraptor 1, then Staraptor 2.
5) Power items (does not affect)
Staraptor 1 holds Power Lens with 151 Speed, Final Gambits Staraptor 2 with 150 Speed. Replacement was in order of Staraptor 1, then Staraptor 2.
6) Paralysis (does not affect)
Staraptor 1 is paralyzed with 151 Speed, Final Gambits Staraptor 2 with 150 Speed. Replacement was in order of Staraptor 1, then Staraptor 2.
7) Stat drops (does not affect)
Staraptor 1 is at -1 Speed from Electroweb with 151 Speed, Final Gambits Staraptor 2 with 150 Speed. Replacement was in order of Staraptor 1, then Staraptor 2.
8) Pledge Swamp (affects)
- Staraptor 1 is under the effects of Pledge Swamp in a Double Battle with 151 Speed, Final Gambits Staraptor 2 with 150 Speed. Replacement was in order of Staraptor 2, then Staraptor 1.
- Also tested: same situation, but Pledge Swamp expires on this turn before the next Pokémon is replaced. Replacement was in order of Staraptor 1, then Staraptor 2.
Third round of tests: various Pokémon because I needed different modifiers
9) Chlorophyll / Swift Swim / Sand Rush / Slush Rush / Surge Surfer (does not affect)
Staraptor with 150 Speed uses Final Gambit on these Pokemon in their appropriate Speed doubling conditions.
Alolan Sandslash – 100 Speed (Staraptor replaced first)
Kantonian Sandslash – 100 Speed (Staraptor replaced first)
Surksit – 100 Speed (Staraptor replaced first)
Jumpluff – 130 Speed (Staraptor replaced first)
Raichu-Alola – 130 Speed (Staraptor replaced first)
10) Unburden (does not affect)
Staraptor with 150 Speed uses Final Gambit on 102 Speed Slurpuff with Unburden active. Replacement was in order of Staraptor, then Slurpuff.
11) Quick Feet (does not affect)
Staraptor with 150 Speed uses Final Gambit on Mightyena with 120 Speed affected by Quick Feet. Replacement was in order of Staraptor, then Mightyena.
12) Quick Powder Ditto (does not affect)
Staraptor with 150 Speed uses Final Gambit on non-Transformed Ditto with 102 Speed holding Quick Powder. Replacement was in order of Staraptor, then Ditto.
13) Slow Start (does not affect)
Staraptor with 150 Speed uses Final Gambit on Regigigas with 160 Speed affected by Slow Start. Replacement was in order of Regigigas, then Staraptor.
Fourth round of tests: changing the Pokémon's Speed stat by non-modifier means
14) Trick Room (affects)
- Staraptor with 151 Speed uses Final Gambit on Electrode with 220 Speed in Trick Room. Replacement was in order of Staraptor, then Electrode.
- Staraptor with 151 Speed uses Final Gambit on Raichu with 130 Speed as Trick Room is intentionally reversed. Replacement was in order of Staraptor, then Raichu.
- Staraptor with 151 Speed uses Final Gambit on Raichu with 130 Speed as Trick Room naturally ends. Replacement was in order of Staraptor, then Raichu.
15) Using Speed Swap (affects)
Smeargle with 139 Speed uses Speed Swap on opposing 150 Speed Staraptor. Staraptor uses Final Gambit on Smeargle. Replacement was in order of Smeargle, then Staraptor.
16) Transform (does not affect)
Smeargle with 139 Speed Transforms into Electrode with 220 Speed. Staraptor with 150 Speed uses Final Gambits on Smeargle. Replacement was in order of Staraptor, then Smeargle.
17) Fainted Forme regression (does not affect) (see notes below in this post)
- Megas – Metagross with 134 Speed prior to Mega Evolution, 178 Speed post-Mega vs 150 Speed Staraptor (Metagross replaced first)
- Ultra Necrozma – Dusk Mane with 129 Speed prior to Ultra Burst, 181 as Ultra Necrozma vs 150 Speed Staraptor (Necrozma replaced first)
- Zygarde – 50% with 115 Speed, Complete with 105 Speed; 112 Speed Pokémon (112 Speed Pokémon replaced first, then Zygarde)
- Darmanitan – Normal Mode with 110 Speed, 70 Speed in Zen Mode; 102 Speed Pokémon (102 Speed Pokémon replaced first, then Darmanitan)
- Meloetta – 121 Speed in Aria Forme, 162 Speed in Piroutte Forme; 151 Speed Pokémon (Meloetta replaced first, then 151 Speed Pokémon)
- Greninja – 174 Speed prior to Transform, 184 Speed post-Ash-Greninja; 178 Speed Pokémon (Greninja replaced first, then 178 Speed)
- Wishiwashi – 51 Speed in Solo, 41 Speed in Schooling; 42 Speed Pokémon (42 Speed Pokémon replaced first, then Wishiwashi)
- Minior – 146 Speed in Core, 80 Speed in Meteor; 102 Speed Pokémon (102 Speed Pokémon replaced first, then Minior)