with the 24 valve in sig. water meth made 51hp on my truck, egt's dropped 150 degrees, that is with a 50/50 mix. im running snows stage 2 and have the pump cranked to the max with 2 of his largest nozzles (625ml?), oh ya, i just blew my head gasket today and im only running 32-35psi. so yes in large quantities with higher doses of methanol its hard on headgaskets.
-20 degree washer fluid is approx. 20-30% methanol, hardly a seat of the pants gain in power but still a nice egt drop. if you are using it for towing you will be injecting a lot less water into the motor and you shouldnt have head gasket problems.