Those temps are fine; it's more often too much paste used, not too little. I wouldn't mess with your heatsink. Most small water cooled setups only lower the temps a few degrees. Depends on your room temp also.
During idle, only one core should be active. The adjacent cores may have some residual heat from the active one. It's normal for some variance in temps. You won't get rid of it all by reinstalling the cpu multiple times.