So let's say I have a Gengar with a 412 base speed. Would his evasiveness in any way be affected by that? And if not is there any stat that directly effects evasiveness? thanks for your help!
No. Speed and Evasiveness are two different stats so your Gengar wouldn't be affected in any way.
And there is an actual evasiveness stat and it "effects" evasiveness, but if you mean any other stat, then no.
Hope this helped!
Source: Knowledge
A: a Pokemon's max base speed is 255; A Pokemon's Speed Stat can be 412.
B: No, speed doesn't affect evasion.
Source
Hope I helped!
No speed in no way affects it and
other than evasiveness itself [obviously.] there is no stat that effects evasiveness. but raising accuracy can counteract evasiveness.