what caliber?

First- I'll assume you're referring to a hot rod tune. There are many tunes for these guns and not all of them are for max power, but here goes:
These are what I typically see when tuning -97's for a bit extra power WITH smooth firing-
.177 can go to 15 normally and some exceptional guns can see 16 with 7.9 gr pellets.
.20 caliber can see 15.6 to near 17 ft/lbs using 11.6 gr pellets.
.22's can do 16.5 to 18 ft/lbs.
Pushing the gun to the higher limits in each range noted above does increases snappiness a bit.. though still twangfree. The lower end tunes of each of the above ranges produce very civilized guns with better than factory power... and cheaper than a VMach tune. Such a (max)tune also increases cocking effort as the spring must be stouter.
Just remember there are different levels of tuning and that will be reflected in the cost and sometimes shot cycle of the gun. Too many folks see the word "tune' and immediately assume a gun has been kicked up a couple ft/lbs. I'd bet in at least 50% of the cases, it simply isn't so. Tuning is about making the gun shoot better..although 'better' can mean different things to different people. RB