{disclaimer: I never did well in physics}
I imagine tapering the end would increase air speed slightly, as well as increase air temperature ever-so-slightly. If you tapered it off too much, you'd probably start losing air (i.e. pressure in the tube not letting as much air in through the front end) but if it was just a slight taper, I doubt it would make a whole lot of difference. If anything, I'd widen the 'neck' where the hose points at the disc, to "cool" the air before it hits the rotor. I don't think it'd make any measurable difference, but it would make
me feel better.
