Is there a rough/easy way to estimate how this move affects a pitcher's numbers?

Look at good teams in each ball park. Look at those players on those good teams. Avoid guys using crappy defenders, catchers etc.

Stadium (-) numbers for 1b will help a pitcher's batting avg given up line (lower it). All other stadium categories (2b, 3b, LF, RF) will affect pitcher's batting average and slugging given up.

The extent is difficult to predict.

I like pitcher's parks because outstanding pitching seems difficult to find and develop. Also, since pitchers age more gracefully in the sim, owners tend to hold on to the great pitchers until they are in their late thirties so they are rarely available unless you overpay. Also, it seems that finding a decent hitter or fielder to plug a hole is a lot easier than finding a good pitcher to fill a hole. So, on average the quality of the lineup/Fielding is better than the quality of pitching. The extent of this depends on the World you're in, but in my experience, it is nice to give my pitching staff a stadium boost. Even if my hitters get nerfed in the process.

I'll just echo tlowster's observations. Your pitchers will, on the whole, pitch over their heads a little bit because batting average for everyone goes down. And yes, it nerfs your own hitters in the process. You have to be happy with playing lower-scoring games.

I have now switched to mostly neutral parks. I thought I was being clever and cute by playing in Wichita and targeting leftie pull sluggers to take advantage of the difference between the Of walls in LF and RF. There are many types of talented players and to limit yourself to one type of player is to immediately put yourself at a disadvantage.

I still think great pitching is more rare than great hitting, but I have learned that a good defense can turn mediocre pitchers into all stars and good pitchers into great pitchers from a numbers perspective. No need to nerd my hitters in the process.

Park factors are huge so I try to keep it easy on myself by removing them from the equation for 81 games.

I always play in pitchers parks, just because it helps your players' fatigue over the course of the season.