All Aquacomputer D5 Pumps have a minimum speed of around 2000 RPM. This is intended by Aquacomputer.

~2000 RPM is the minimum RPM intended by Laing/Lowara. Are you sure that your EK D5 pumps runs at 800RPM and this is an "original" D5 pump?

Anyway the pressure at 2000 RPM is less than 100 mbar so mostly doesn't make much sense to use lower RPM. At 800 RPM pressure would be around 14 mbar - I don't think you will get any flow.

ek pumps go down to 800, well.. 400 RPM effectively. you can see the rotor spin and yea, it only provides a trickle of water.
I have an unused EK FLT360 on the desk with a D5 and a simple pipe from outlet to fill port, and it's a good display piece :p when set to minimum, even with a straight connexion (no rads, no blocks, just 30cm of tube), there's barely any flow.
but in real use, 400 rpm does nothing. The value you gave is really a minimum. below 1000rpm (2000 reported), temperatures go up quick.
And the pump is not noisier. it's still very slow.