How does fatigue impact pitching performance? Does it gradually wear away ratings throughout the game? If two identical pitchers are starting and one is 93% and another is 100% will the 100% more likely to perform better?

I had a guy start this season at 86%, and give me 7 IP, 2 H, 0 ER. Normally, I have his pitch count at 90/105. I lowered it to 75/85 before this start, and he went 83 pitches, and was back to 100% when his spot came up again.

I wouldn't recommend it. But if you have an emergency, and your choice is to start a quality SP at 93% or use a lesser pitcher at 100%, I'd go with the quality SP. Just adjust the pitch count accordingly.

Yes, a guy starting at a lower fatigue percentage will perform worse than a guy at 100%, all other things equal. There is in-game fatigue. I don't know if we've ever been told if it's gradual (ex. a guy loses 1% effectiveness every pitch) or if there's a certain pitch limit a player can get to before a quick performance drop-off (ex. a guy can throw 90 pitches, then lose 4% effectiveness after each pitch after that), but I'd imagine it's closer to the latter, since that's much more realistic.