[JustinP10]

I wouldn't think you'd lose power in the summer due to timing, only to the higher intake air temps (less dense air). I live in Arizona, where we can see temperatures as high as 120 degrees fahrenheit (about 49 Degrees C). With my timing at 19 degrees and running 92 octane gas I didn't hear any detonation, but truthfully, I was probalby pushing it. I never really got on it when it was that hot out (car runs like crap when it is that hot) so i don't know how it'd do at WOT, but under normal driving conditions I didn't hear any detonation, that's about the only reason I can think that you would lose power.
The biggest downside i see to the advanced timing is the extra money spent on gas, since you require a better grade once you advance the timing.