first question depends on what type of coolant you plan to use. I personally use Prestone green antifreeze, at a 80% distilled water, to 20% antifreeze mix, so mixing metals for me, does not matter at all.
the TR4 water block from XSPC, has the highest thermal rating out of the water blocks made for the TR4 CPU's.
yes, the cheap $24 EK TR4 water block, has really poor performance on the TR4 CPU's, that is the reason for it being so cheap.
Best performing TR4 blocks are Heatkiller TR4, XSPC Raystorm Neo TR4, EK TR4 depends if its older one then these are poor performing blocks
...have been using the Heatkiller TR4 (Nickel-plated version) on a 2950X and MSI Meg Creation @ 4.25 GHz since November. Great quality etc. and absolutely no complaints on temps, idle temps in mid-20s C and full load-temps rarely even get to 60 C. In fairness, system has dual pumps and dual 360 rads for the TR, and I covered the IHS w/ MX4 end to end.