Speed Infusion 11 does not work

Uh what are you smoking? It works for Buccs. When I use Barrage with just my level 11 SI it seems a bit slower than if I Barraged with SI AND Booster.

Because knuckles are fast (5), which with either speed infusion or booster alone does not reach fastest (2). With both of them you only receive the benefit of one more speed level, which is why your barrage is slightly faster, but you don't get the full effect of -4 speed levels because of the speed cap. That's what the TS is trying to prove.