There's a lot of spare parts in the typical o/cer and watercooler's closet. If you "upgrade" to the part of the day you'll go broke. Bill's comment on total system performance is well noted, and I will add my (anecdotal) test results here. They differ from Bill's in that:
1)My flow rates are +/- 6%
2)My temperatures are less precise
3)I was not isolating the waterblock so there is a heatercore for heat exchange with air rather than a chiller.
Take the numbers as illustrative rather than quantitative, please.
The test setup was a Maze2 (newer rev) 1975 Caprice heatercore with 2 Panaflo H1A fans, Eheim 1250, 1/2" silicone, and a ball valve to adjust flow rate. Overall conclusion was that you must keep flow rates over 1 GPM; with the pressure drop of the flowmeter and the 1250 I couldn't get to the higher flow rates where Bill sees no improvement. I would suspect, however, that if the wb performance isn't improving but the radiator's efficiency is decreasing with ever higher GPM, then the delta T may actually rise again.