This might sound dumb that I bring this up, but up until about 5k miles, my auto 2005 GTO's tires would squawk at WOT when shifting from 1st to 2nd. Now it doesn't at all. Is my tranny not as firm? or are my tires now just "broken in"? I'm a little concerned. I also feel a very slight lag and lurch when gently tapping on the gas in city driving.

Well thats normal for an auto, at least the ones I drove had a slight lag to them as the tranny has to compute what you want it do.

But the not squaking tires could be a number of things, tire condition, road surface, air temperature, engine condition...as long as it still feels strong, then its most likely that the tires have more grip because they have warmed up or are on a sticky road.

I tried it on many roads and all different temps, so I'm stumped

Did you turn off traction control.If not do so.Mine don't squeel anymore,but the tires lay down rubber.I suspect your tires are worn enough that they have more grip and the tourk converter and traction control are brokeen in now.The tires of the old days had different type of rubber and construction and would squeel even going around a corner.Todays tires have sometimes 3 diffeent types of rubber compound in one tire for enhanced grip.They'll burn but not squeel.You can get more noise by increasing the tire pressure sometimes.

Your tires will tend not to chirp after they get some miles on them. There are other factors which may actually be causing the engine to make less power as well, which includes the hotter summer temperatures. Our cars tend to heat-soak easily, and performance suffers because of it. The PCM also "learns" your driving style, if you only get on it once in a while it goes to a less aggressive shift map. If you beat the snot out of it often, the PCM goes to a more aggressive shift map to suit your driving style.

I have roughly 4,900 on the odo and when my '05 shifts into 2nd at WOT it hits hard and gets all kinds of squirrelly.

While we're at it, will one of you more informed answer a question for me?

How much horsepower can the auto tranny handle before turning into a stack of Doritos? Curious because I'm getting ready to do some mods that should take the car to 450hp or so.

Thanks in advance.

You should be fine at 450 I would think.