I think on a stock vette be it C5/6 you would get about 10hp out of a good tune.
It might feel like more then that in a automatic as they can change the torque management
and change your shiftpoints etc and this can make a big difference in drivability.
If you are adding a new cam or other power adders then a tune can help get all the HP out of
your new components that you would not without a tune (full optimization).
The PCM only has so much adjustment that it can compensate with beyond that we need to use
EFI live or HP Power tuners etc to tweak in the timing, fuel maps, torque management etc.
I think stock automatic cars have the most to benifit from a good tune.
There are other benifits also like killing CAGS and turning the two radiator fans on at earlier temps
to keep things cooler, kill error codes for cat's etc.
$600 sounds a little steep for a tune, but then that does depend on how they are doing the
process by either road tune and/or dyno tune or there is even mail order tunes now that should
save you a few bucks.
Here is a writeup by "meadowz06" on Corvette Forums, hope it helps answer your question.
Hp tuners dyno tune done on my 02 Z06!
I will post the graphs later. This was done on a very stingy mustang dyno. Baseline was 337hp. We ended up tuning the car very conservative, no timing put in at all, and we ended up with 352.7. We left somemore on the table. I am ordering a 180 stat and we are going to run somemore timing in the future. Again, this tune was with NO TIMING ADDED AT ALL! We took out the torque management, cut the fans on earlier, removed the skip shift, and smoothed out the air\fuel ratios. The guys at SPEEDSOUTH in Pelham, Alabama were\are great. They did a great job with the car. It runs a ton smoother and pulls harder on the top end. Much more driveable. I will post the pulls when I get all of them in a email. Completely stock 02 Z06 with the zip tie mod and stock filter.
02 Z06's can put down in the neighbourhood of 370 rwhp with the airfilter mod, TB coolant mod and a tune. The AFR stock is in the 11's-leaning it out to 12.5 helps a bit. The 01's will probably see about 10-15 rwhp less. Were very close to being setup for mail order tunes-cost is $250 canadian.