Prop pitch speed
The formulas for estimating pitch speed are just that, an estimate. You'd have to do a serious break down analysis of the whole set up including blade airfoils, twist distribution of the blade, in air RPMs, and of course airframe drag. The reason why you are able to go past this estimated pitch speed is because the formula only deals with blade angles, and the airfoils on the blades are creating lift. The faster you spin a prop, the higher the reynolds numbers, the more lift the blade produces. You also have aerodynamic unloading of the prop in air. It can increase quite a bit once up and zipping around.