My minor league parks are either relatively neutral (RL/HiA/AAA) or extreme pitchers' parks (LoA/AA). If I have a hitter who is ready for A-ball ... since he's a prospect I actually care about, we'll say he's ready for Hi-A with the possibility of AA ... is his development affected by his performance, or is it only based on playing time?

Example: I give a hitter 500 PA no matter what level I place him. In Lo-A, his numbers are so-so ... he's v.good for the level, but the park brings his numbers down. In Hi-A, his numbers are v.good ... he's good enough to play there, and the park effects are negligible. In AA, his number are mediocre ... he might not be ready for AA, of course, but even if he is, the higher level of competition and the extreme park effects bring the numbers down.

Will he develop at the same rate no matter which park I put him in? Should I just ignore park effects and put him where he's ready, paying attention to ratings rather than stats (which I'm doing anyway since this is the minors)?

I believe the answer you are going to get is that stats don't matter in the minors...but we'll see

Agreed. He can play on a sandlot. The important thing is that he's playing.

yeah IPs and ABs seem to be more important than some stats in a minor league park

Playing him in an extreme pitchers' park, however, can limit AB by not going through the order as often. The flip is also true- putting a pitching prospect in a pitchers' park can extend their IP on the same pitch count. I don't know whether the development sim uses IP or Batters Faced to represent playing time (similarly, is it AB or innings played for hitters?), but if it's IP/AB this could matter.