Listen to Grundy, he really knows his stuff, well except engines, wheels, bodykits and a few other areas of motoring
On a serious note, the usual way to quantify some power gains is via Dyno runs. Is this something they can do? Can they provide a detailed list with what parameters their tune change? Can they disclose what method / software they're using? As others have said before UpRev is the most known / proven tuning software. Personally, I'd steer away of anything else for this platform, except in crazy FI builds where standalone ECUs can do the job a lot better, see Syvecs, Haltech etc