Honestly I see 49 as maybe being the most affected.
I havent played 49 that much but from what ive seen its all burst damage. I know people say 19 is all about placement and movement skills but 49 may rival that.
I mean at 49 if you're in the wrong place at the wrong time, its game over for sure. From playing 49 mage and being slightly underpowered to playing 49 shaman and being on top of the food chain, everything dies in like 10 seconds.
Either you're killing someone in 10 seconds, or being killed in 10 seconds. But as far as general PvP goes, everything happens ridiculously fast at 49.
That being said, HP will drop a good bit, but damage wont. Thus, making stuff die even faster. Goodluck as a 49 mage getting even a scorch off before a ret pally or shaman ices you in that amount of time.