Here's a graph of the magnitude of <Vd,Vq> in blue, and the rotor flux speed in reddish-orange. I let it spin up pretty fast, and then pushed on the flywheel with a plastic cup, which melted the cup, but notice how nicely it tracked it! There was no filtering at all on the magnitude of <Vd,Vq>. But my "magnitude" algorithm didn't involve a square root, so I need to double check that it is accurate over the full range:
So, you really ONLY have to pay attention to the RPM for knowing when you need to boost. Load makes absolutely no difference. By the way, you could probably get a loosy goosy sensorless working with ONLY using the magnitude of <Vd,Vq>. At least good enough for use with a 4 or 6 count hall effect thing.