normally you would tend to look for 0 toe or toe in on the rear. Unless you are building a drift machine of course. I'm also kind of surprised you were able to get -2.5 on a stock suspension.
The stock bolts normally won't get you much past -1.0ish in my experience.

I run 1/4" toe out on the front with 0 toe in the rear. Help's turn in and allows you to put power down coming out of a turn..

Some shops give you a sheet that doesn't really match whats on your car. Is this a good custom shop or just some random place?

 

I would guess that they "kind of made up" those numbers. With -2.5 degrees you should be able to see pretty siginificant angle from the front of the car looking straight on at the front wheels.

see if You can beg, borrow or buy a camber gauge from someone local to you.

 

I run -2.7 up front on my STi, but that is with coilovers and camber plates. Both my stock camber bolts and the camber plates are maxed out. You would definitely notice the camber visually as stott said.

Here are a couple of pics from my last track day to gove you an idea of what it looks like visually:




FWIW, if you really do have -2.5 camber up front, I would suggest running at least -1.0 out back, if not slightly more than that.