I did a quick analysis on the ESPN data - seems like only about 10 teams showed what I'd consider to be a significant difference in Runs vs LHP and RHP (I used Runs/AB as my metric).
My concern with adding the variable is that there is no way to isolate:
1) The quality of the opponent - perhaps they played against shitty lefties vs great lefties (more confidence that the RHP data balances to league average)?
2) Park splits - perhaps they faced lefties more often on the road or in better/worse hitting parks?
I would think by this time of year that there wouldn't be massive skews but it gives me pause.
I'm hoping to do a larger analysis in the offseason - maybe I can incorporate it then to see how much it adds...