A real test would be before/after 1/4 mile runs at local track. If it really gains 30hp, timeslip should drop three tenths of a second. Of course, the rest of the car would have to remain stock, no removal of spare tire or seats, no other mods to engine, full tank of gas, and stock tires.
Run at least three runs before, and repeat after "tune".
Reason I'm doubtful is back in the day there were "tune" chips for Camaros, Mustangs, and many others. Historically, they all cheated on advertised gains. Its been that way for 25 years.
I am not here to cheat anyone, but I fully understand the concern. Exactly why every single dyno sheet I own is 3rd party. People don't trust tuners, it's quite common.
I believe within that 100+ page thread on mazda6forums there was a guy who did track 1/4 runs to test that and took an entire second off his time (if I remember right). The slips are posted in that thread.