RX7 FC Meth Injection?
 

So I'm thinking about getting a turbo II, and am considering putting it on meth injection, not for power gains, but rather to get the engine to run cooler. Anyway, just wondering how often you go through a gallon of meth?

Depends on your setup, this may help you find what you need:

. https://www.rx7club.com/auxiliary-injection-173/

As previously mentioned, it really has a lot of variables. Nozzle sizing largely determines demand, their management system and engine power and application and those things determine how fast you go through fluid.
First, methanol is only practically mixed in something less than 50/50 with (distilled) water, otherwise it's a fire hazard.
Second, if your T2 is stock or mildly modified, your nozzle size will tend be relatively small.
Third, with a mildly modded car at stock boost (mostly for cooling) your system could be pretty simple. Probably just a boost-activated system similar to mine. Again, fluid usage will be pretty low.
Fourth, if your aim is just for cooling you could also just run a water-only system (WI). And just like a methanol/water (AI) application, you also get knock and carbon control.

For comparison, my FD makes a little less than 350 hp (maybe 275-300 at wheels) and other mods listed in signature. I use a M3 nozzle, water-only, boost activated system with pump activation at about 1 psi. The stock windshield washer tank is the reservoir and it holds, IIRC, between 2.5 and 3 liters. Starting with a full tank of gas and a full tank of distilled water, and depending on how hard I'm driving, I can use between 1/2 and 3/4 tank of gas before I get a low-level light and have to re-fill the water.

What water systems do you recommend?

There are several suppliers that are available....Aquamist, Coolingmist, Snow Performance and others. And some of the individual components are available from multiple sources.
My 'system' is very simple and I bought the components ala carte, I think I bought the line, check valve, in-line filter, nozzle, 90 Deg. nozzle fitting and pump from Coolingmist. A float switch to tell me when the level was low, and the Hobbs switch (boost switch) was from Aquamist IIRC. Small Red and Green LED lights from a local Radio Shack. Red light illuminates when water level is low, Green light, when pump is activated.
It's been nearly 10 years, but probably < $350 back then.

If you're interested in the more technical aspects and/or more sophisticated multi-nozzle, progressive and managed systems there is an Auxillary Injection section here with ALOT of info in the stickys and even more available in searches. If you want more info on mine in particular (pics reloaded at the end) https://www.rx7club.com/auxiliary-in...ion-fd-959565/