Well, here's a quick comparison: AnandTech Bench q8300 vs i5-2500k
, but I would say the gaming scores specifically are more exaggerated at Anandtech compared to most everything else I've read.
It's still a pretty good boost if you go to a 2500k though... hard to put an exact number on it, but I would say anywhere from 10% up to 50% on some CPU dependent games.
Ivy Bridge and hopefully Bulldozer will be a bigger difference than that.
EDIT: At those temps you won't shorten the lifespan by any noticeable difference. Voltage and heat are both responsible for CPU degradation, but heat is by far the bigger culprit. By the time it goes it will be utterly obsolete (still another 5-7 years easy, probably more). I would say you could go higher on the overclock if you wanted, but an aftermarket cooler would do wonders or water cooling if you are so inclined.