If you're at 2lbs minus gear then I think you're OK as long as you don't put heavy gear in it. You should come in under 4lbs. The Fox 40 has alot of pull. "Spool up" basically means the speed of throttle transition. With a 12-4 prop...there is alot of load on the engine and when you hit the throttle there will be a measurable delay in reaching max RPM which is bad news for 3D flying. You need quick throttle transitions. An 11" flat pitch prop is as big as I'd go with the 40. If you're light enough....a 10" prop might be to your liking. I can't say for sure since I don't run any .40 engines.