BTW: A 20oz tank DOES significantly affect a small plane. Alcohol is lighter than water, but water is still 1 POUND for 16oz. Methanol is about 80% as heavy as water, so we can pretty safely say that 16oz of methanol weighs about 13oz, or more closely, 0.8oz per liquid ounce. A 20oz fuel tank would be pretty close to a pound... that's more than a 10% increase for my Goldberg Extra.
I carry 28oz or 1.4 pounds of fuel, plus another half pound or so of smoke fluid, so I'm getting up into the range of a 2 pound variance from takeoff to landing.

How well would your heaviest plane fly if you added/subtracted 2 pounds from the nose?