Isn't the WW a high flow block that requires a higher flow pump? The lack of head on the pump may explain some of that...
Oh, and the TIM shouldn't really find a 2-3C diff from one paste to the other should it? Are you mounting block with enough EVEN force to push it flat against the die?
One other thing you may have overlooked though.. (serious). When taking out a block, you may have loosened some crud. The crud floats around in a system. The WW is vulnerable to crud in the fins over the die. If you had some crud, (growth particles or loose teflon tape) it may have become lodged in the WW. Change your water and clean the system out. Check for goop in the fins.
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