From what I've seen of BillA's graphs (or someone else at OC), the difference can be quite large, for similar designs, but it depends on a number of things: for example flow rate.
At 300gph, the difference between Alu and Cu is 3 deg C (core temp). The thing is, most people aren't running at 300 gph, and the difference is far greater at lower flow rates (10 deg C plus).
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