15 in nowhere near the upper limit. The stock tune sucks badly, especially on the earlier cars. I got 28 real world HP out of the supposed 16 HP Procal on my last car. A canned tune can be really outstanding like that or it can disappoint, though, because it really depends on how your car is to start with. The tune on my 14 wasn't as big an improvement, because the factory tune was way better to begin with. Any tune that takes your car's information and informs tweaks to the tune will be better. For commonplace applications like a stock MGT, though, a canned tune will very likely give you excellent results.
2014 Premium GT, SGM, Brembo, Auto, Tech, Comfort, etc. GT500s, Steeda UCA, VMP auto tune, 100 shot on BBR tune, best et: 11.38 at 120.85
2012 GT, Candy Red, Brembo, 3.73, Tech, Comfort, etc. Procal tune, Roush upper, UMI LCAs, Steeda red bracket, GT500s, 28X10X16 Hoosier slicks, best et: 11.91 at 115.23 RIP