throttle stick and RPM are far from linier, RPM and power are also far from linier!!!

Also, power to weight depends on altitude, humidity, all sorts of things! on big cowls like the yaks have, its important to have a good distance between the front of the cowl and the prop, you loose sugnificant thrust if its too close. a smaller, streamlined, extra style cowl has have a smaller gap. This isnt just apparent on RC models, but also on full size (see a pic of a full scale yak!!)

and whats quite important, and what most people forget, is DRAG! if your going at 0mph, say in a hover, drag is no big problem, but in a strait line it is. If you double the drag, you have to x4 the power. 3 times the drag, 9 times the power, its just squares.