While it's true Nitrous would cool the intake charge, it will also make for an even hotter combustion process. Nitrous goes in cold, but it's an accelerant. Don't think of nitrous as a suplement to meth, it's strictly a power adder when injected into the motor, and should be treated as such. Either way, it'll make for an excellent pick-me-up in torque/hp.
Meth is like anything else, when used correctly produces the results it was intended for. Meth is fuel enrichment, not a power adder. Lower IAT's are a secondary benefit to using meth. Some people try to tune WOT basing it off added Methanol, which you can get away with so long as those variables never change. A car *should* be tuned without meth, and then meth added after the finalized tune, jetting accordingly. I can say as an eye witness that meth on the M90's boost level is kind of a waste. Not quite enough boost to justify it, unless you're trying to be very aggressive with spark timing. Again, if it's jetted properly, and you have a decent meth kit, and don't tune your car to rely on it, meth is good shit!
Is it possible just to tune the car so that the only benifit is teh lower IAT and not utilize the fuel aspect? Granted the m90 does not produce a huge amount of boost but it does build heat like any typical TS or roots. I am very happy with the M90 but when at track it loses about .2 to.3 just from being hot.