To know what the gains will be, you have to know where the power gains of methanol come from. It doesn't exactly come from "cooler" air. It comes from the fact that you can run more TIMING with cooler air. The methanol-cooled air itself adds exactly 0 horsepower. But the gains from 10+ more degrees of timing.....well....that can be a lot.
Therefore, the gains will depend on the location of your IAT sensor and your current tune. If your IAT sensor is located before the blower, the tune will not/can not add any timing because it isn't seeing a noticeable difference in the IAT temperatures, and you will see little or no gains. As for the tune, if it's a VERY conservative tune that doesn't add much timing even with MUCH cooler temps, you won't see much difference either.
To be effective, the IAT sensor MUST be located on the boost side of the blower and well after the methanol injection point. I honestly am not sure what you'd do on a FMIC car. If you had the IAT sensor after the intercooler, there wouldn't be a large delta temperature with and without meth for the tune to base timing on; but if it was pre-intercooler, the computer wouldn't know that the car was intercooled, and you'd lose a lot of power when not spraying meth. I suppose the ideal situation is to have a IAT sensor before and after the intercooler, but I'm not sure if that's even possible. Maybe someone with more knowledge about this can chime in.
Then, the tune must be designed in a way that is 100% safe with no methanol (i.e. maximum possible temperatures and corresponding timing curves), and add timing as the temps come down from those base-line (maximum) temperatures. The AFR should be set to a safe level (11.0-12.0 depending on the setup) without any methanol.
THIS IS VERY IMPORTANT. ONCE THE AIR/FUEL RATIO IS CORRECT WITHOUT METHANOL, DO NOT ADJUST IT UNDER ANY CIRCUMSTANCES. When you spray methanol, the AFR will show pretty rich. However, this is misleading as the stoichemetric ratio of methanol is lower than gasoline, so it actually comes down very little. If you adjust the AFR so that it shows correctly WITH methanol, then it's going to go VERY lean if the methanol doesn't spray. This is where the horror stories of methanol come in, and how I lost a motor as well. BUT, when done correctly, there is a 0% chance of things going wrong, because if there is no methanol, the tune behaves as it would with no methanol (which should be done rather conservatively).
Now this increase in timing from the methanol can be done aggressively or conservatively. For someone with a blower at 10 psi with an intercooler, you're probably around 14-16 degrees of timing currently. With the methanol, you can have it increase timing rapidly with small decreases in temperature up to a higher total timing (24-26 degrees is very possible), or you can have it increase the timing slowly with larger decreases in temperature to a more moderate max timing (18-19 degrees). Or you can go anywhere in-between.
On my car (Eaton blower, non intercooled, 8-9 psi with PI heads and blower cams), I have two tunes, a street and race. On both tunes, without any methanol, it was tuned to a safe 11.8-12.0 AFR across the board.
On the street tune, the max timing is around 18 degrees, which sounds like a lot (because it is), BUT, it will only put in that much timing if the IAT is 130 degrees or less (i.e. I'm putting the meth to it). As the temps go up, the tune pulls timing pretty quick, around 1 degree of timing per 10 degrees of temperature; at 230 degree IATs (which happens easily in the summer with no meth), the timing winds up around 11 degrees max, which is very safe.
On my race tune, the max timing is at 24 degrees. Similar to the street tune, it will only add this much timing if the temps are 110 or less, which really only happens at the track after a meth-saturated burnout or multiple pulls on the street. To get back to a safe level, it REALLY pulls timing as the temps go up over 140. As with the street tune, it's back to very safe timing levels by 180 degrees or so.
Someone asked about the power gains. I can tell ya, going from 11 degrees of timing to 24 on a blown car is a lot like a slow-occuring shot of nitrous.
When I spray it on a really hot, heat-soaked motor, my car feels like a turbo car as it starts adding in the timing. When I spray on a cool motor that will already have the timing added in....well.... there's normally black marks left on the road. As far as an actual documented gain, I showed very little gains on the dyno, around 20 horsepower. BUT, I have recorded a 7+ mph gain in the 1/4 multiple times now, which equates to at least 70 rwhp. (Keep in mind this is a non-intercooled blower).
Oh, one last thing. Don't be afraid to hit it with a LOT of methanol. I was told by Snow, Devil's Own, several 'knowledgeable' tuners in my area, and several others on various forums that I needed a 3-5 gallon per hour nozzle for my setup. Well, a few guys on the TCCoA and a rep from AIS stood me down that I needed MUCH more than that. Well...after taking the long way around and being stubborn, I found they the TCCoA guys were all spot on the money. I'm spraying mine with 14 GPH all the time, which looks like a metric crap ton (and it is), but I have documented that that's what runs the best at the track.
Wow, that reply was longer than I anticipated. I hope I haven't confused anyone, and feel free to ask any questions about it.