With the "canned" Predator 91 octane tune my car put out 378 RWHP. The same car, on the same dyno, on the same day put out 411 RWHP after the CMR tune was done on the dyno. I was pretty sure that since they were able to tune the car in the relative ambient conditions that it lives, they may have been able to tweak on the settings as they were going along.
But I'm not saying it's better than a custom tune by email with you sending logs to the tuner and them setting a baseline, then tweaking until the second and third set of logs come back to them. I remember the shop doing a baseline, then three pulls while they logged, then tweaked, my tune.