I have/had that setup (the wc parts are now in a box in the attic but I still use the case).

It did exactly the same thing to me - if you leave the fans on auto speed.

Try setting them to something like 5 or 6 manually and see how your temps do.

That dropped my idle temps by about 5C. Load temps still sucked. It's the pump I'm sure of it - the flow rate is dire at approx 0.5gpm. The "upgrade tank" puts a DDC3.2 pump in there instead which would help A LOT.

HTH
J