I agree with what's been said about the sentry guns. Even from two hundred feet away I've gotten my butt kicked by those sentry guns. From that distance only the soldier really has a chance at taking them out, and they may very well die just from the hits they take the two or three times they peak out to take the shot.
I know on 2fort if an area has a sentry gun, it's pretty much off limits. The demoman's grenades can take them out pretty well, but the range on those grenades is absolutely terrible and won't be able to take out half the sentries you'll encounter.
I think the spy is perfect just as it is though. Yesterday three engineers had set up shop in the sewers on 2fort and we couldn't get anywhere near it and had no spies, so I switched over and on my second try I managed to kill them all and destroy the sentries/teleports, everything. Unfortunately it is not always so easy. I know the ONE time I played the engineer, my guns got blown up once by a spy. Every other time I managed to kill the spy, repair my stuff, didn't even hardly have to move and stayed there for a good long while.