Generally, MOST pumps should be at 100% at ALL times, for maximum cooling performance, if that is what you are after. Fans can be variable speed based on thermals. Your temps are not TOO high, at all, because the max you want to see is 80°C on any single core or on the package temp, but you could probably get that down a bit with the pump running correctly.
Bottom line is, the pump has an EASIER time of running at full speed all the time that it does trying to continuously speed up from a lower speed against the thick, viscous resistance of the water. Much like ANY fan, or your car, it is much easier on the engine/motor to maintain a certain speed than it is to GET to that speed from a stop or lower speed. Plus, many pump motors are not even designed to be used with PWM signals or variable voltages. Most of them have traditionally been DESIGNED to run at full speed.
That being said, it does not necessarily HAVE to be FULL speed, and you can determine at what pump speed (IF, IF your cooler has a pump intended to be ok for use with variable speeds or PWM signals) you want to run it based on what kind of cooling performance you are satisfied with, but if you run it at a lower speed you want to leave it at that speed full time OR at the very least, do not reduce the minimum speed to below maybe 25% of the maximum speed as it will be easier for the pump to get UP to full speed from say 3/4 speed that to get up to full speed from 1/4 RPM.
http://www.tomshardware.com/answers/id-2812083/aio-pump-run-100.html
https://forums.overclockers.co.uk/threads/aio-pump-speed-and-fan-question.18760571/
If your temps are within spec at the highest loads you ever plan to run (I'd test it with Prime95 version 26.6 (And only version 26.6) on Small FFT option) with the maximum pump speed set to 90%, then that's fine. So long as you don't exceed thermal spec with the pump speed lower then that is up to you however as I said, you don't want the pump to have to be constantly ramping up from a low RPM to a high RPM when loads change, so try to keep the minimum speed at the highest speed your ears can live with and leave the maximum speed at somewhere between 90-100%.
It would first be best to investigate whether or not the pump on your unit was intended to be used at variable or lower speed settings, or whether by design it was intended to only be run at full speed.