the claims of 125 hp are for forced induction cars using turbos or centri style super chargers with tune upgrades to fully utilize the added cooling effect by adding timing and what not.
They are correct in that it will give you a level of knock protection as it will lower your intake temps. You won't see a lot of gains in power from it until you get your car tuned to utilize the cooler temps by upping the timing and what not.
All in all, if your car is running fine now and not pushing the limits of your car tune wise, I don't see where putting meth on there is going to be all that beneficial for you. Keep in mind also that you will have the continued expense of having to refill the meth just like you would with nitrous
"When I know more, I'll be forthcoming. Or I won't be forthcoming, and I'll be honestly deceptive." - Les Miles