...noise gates are bandaid solutions. its like taking tylenol for chronic headaches - find out what's causing the headaches, and you won't need the tylenol.
as well, noise gates can really mess with your tone and dynamics.
p90s are, apparently, even noisier than traditional single coils.
there are a number of options open to you:
1. use a guitar equipped with humbucking or noiseless single coils for songs where you use a lot of gain.
2. have a guitar tech install professional shielding. this won't get rid of the noise, but it should at least reduce it.
3. install noiseless p90s. the downside is that the noiseless p90s WILL NOT sound as good as your noisy p90s. the only saving grace here is that if you ONLY use the guitar for high gain setting, you won't be able to hear the difference.
4. people like john suhr are working on a system that will defeat single coil hum without affecting your tone. they have a created such a system for strat-style guitars with a trem cavity in the back:
(
http://www.suhrguitars.com/pickups.aspx#bpssc)
but as yet not for guitars that don't.
-dh
(also a fan of both p90s AND the tonebone hot british)