So you really need 1 RPM resolution all over ? At 100 RPM thats 1%, but at 4000 RPM its 0.025% resolution !

You need 150uSec resolution at your top end speed, which rather stops you using the obvious step of putting more vanes on your interrupter to gain response speed, unless you switch in and out a little divide by n stage according to your needs, then you could use an 8 vane interrupter say, and get 8 pulses per rev, as you increase speed you can drop to 1 pulse /rev.

Can you not settle for 1% accuracy over range ?

Then you need to average your results anyway. Assuming your pump is running at a fixed speed for some time, then a rolling average/ exponential forgetter should do nicely.