Originally Posted by thrasherht
D5s are 50psi pumps.
ddc are only 21psi.
opt33 is correct, that is the maximum continuous pressure rating of the pump, not
the output pressure.
For example, if we look at the Black Ice GTX 480 rad
it has a maximum continuous pressure rating of 24.7psi (look at the "features" tab). If your D5 really was putting out 50psi then you would blow your rads, blocks and tubing apart the second you powered up.
Incidentally I believe this is why some people think that the D5 is a better pump than the DDC for high restriction systems, due to it's "high pressure rating".
As to the OP's question:
Yes, you can have too powerful a pump. If the pressure gets too high (see above) then you can physically damage your components. This is unlikely though, unless you really do have an uber pump. Do you have a maximum pressure rating for it?
Also, as opt33 also pointed out (I like this guy!), at some point you will be dumping more heat from the pump than you will gain in performance. Where exactly this point lies depends on the pump and the loop, it would be difficult to work out without testing.
Be aware that the flow and pressure ratings of a pump only indicate two points on it's PQ curve; maximum and zero restriction. How the pump will actually perform on your loop depends on the PQ curve and your loop.