This "problem" on the radiator wouldn´t decrease the disipation of it, but it could be a problem if your CPU block is restrictive and needs high water pressure. If the CPU block is one of these and the pump hasn´t enough pressure, the final temps in the rig could rise a little bit.
However, a lower flow speed through the tubes of the radiator would increase the performance of it, but always if a high pressure pump is conected.
Make some tests and post it here...
Cheers,
friku.