Jim McLennan's post that Livan Hernandez would be equivalent to Derek Lowe caused a bit of a controversy in the comments. His main reasoning for this is that Livan Hernandez faced much tougher competition in the NL East than he would in the West, thus his inflated ERA. While this seems like a good theory, it just doesn't work out in practice.
Take the most extreme example I can think of. The Phillies were the best offense in the NL in 2006, scoring 5.34 runs a game. Meanwhile, the Pirates were the worst in the NL, scoring 4.26 runs a game. Unless I'm missing something, if a pitcher faced nothing but the Phillies, then nothing but the Pirates, he would see his ERA drop by 1.08, a substantial improvement, but this is the most extreme case.
In a more realistic case, shifting from the NL East to the NL West would only mean switching a couple starts against the Braves and the Mets for a few against the Padres and the Giants. The difference between these two offenses in 2006 was around .75 runs per game, which means a difference of three to four earned runs over the course of a season. In Livan Hernandez's case would mean a .15 drop in ERA. It's a decent drop, but it's not enough to keep him from being one of the worst starters in the NL West, nor does it take into account the difference in park factors between RFK and Chase Field, which would at the very least cancel out this advantage.
Baseball is a game of small advantages run over the long term. The difference between a dominant 100 win team and a below average 75 win team is just one win a week. The difference between the best and worst offenses in the NL is just one run a game. Unless a pitcher is shifting from the AL to the NL (because of the DH), there's very little gain to be made by shifting divisions, and what little there is to be gained would be cancelled out by the inherit luck in pitching. While it seems logical that facing tougher competition would result in worse stats, the small game to game differences between teams mean that this just doesn't have much effect.