Ed, if you want to deal in logic, it doesn't make sense that any of us are in this hobby anyways......haha
About half the fuel burned in a heat race is burned prior to launch. Therefore you're down to about 2 ounces making a difference. Yes, it can be noticeable, but realistically it's not within most people's flying skills to truely make a difference (including mine). Maybe I'm just not any good?
If you run the tank dry you'll see a bigger change in trim, but if you compare the trim within 9 laps (lap 2 to 10, the first lap's much slower, so its hard to compare), it's much less of a difference.
Most people's airplanes aren't trimmed within a click or two of perfect anyways, which further reduces the difference of fuel burn.
Although if you're q40's only going 170, it would take you longer to finish a heat, so therefore you'd be burning more fuel in flight......so therefore you'd have more of a change
