for the most part, you want the pump pulling water from a ready supply of water....aka the resevoir. This way you avoid any chance of cavitation of the pump causing lots of bubbles to circulate through your loop.
the ideal loop config would be:
resevoir -> pump-> cpu -> radiator ->...
restricting the outbound water flow is fine. Any pump 10GPM or lower will be fine at .1gpm flow. What you do not want to restrict is the inbound flow, ie from the resevoir to the pump. I recently changed out the piping from my radiator(car radiator no need for a resevoir) from 1/2"ID tubing...
for most people the Eheim 1250 is more than adequate, along with using 3/8"ID tubing.
Resistance going through your system is the main point. Resistance is measured in head/ft. The higher the resistance the lower GPM rate you get. Resistance depends on a few things. How many waterblocks...
Just a little comparo between the two pumps....
Pump I have now
link to Mag9 pressure curve
Pump I just bought
link to i40rt pressure curve
Im giving up about 2.1GPM flow for almost double the max head/ft. I think it will be worth it. Im pretty sure that overall flow will improve...
all i had to do was remove that black thing, and the 2 mounts were already there. just screw the two posts in and slide the waterblock on and follow the install...
washer - spring - washer - nut (on both sides) and then just alternate tightening....
piece of cake.
-Vi