If your mount was really the problem LinX would shut down your PC. (well you hope!)
With the H50 high idle means your ambient is high or your fans are not pushing enough air through the radiator. Delta is nearly the same both ways. (again unless your TIM is inefficiently placed)
If the air inside your case is significantly warmer than your room temp your idle temps will be high. Your case should be (at worst) <5C higher than your room temp. A well ventilated case such as the RV02/FT02 will approach ambient! It's also important that your case does NOT recirculate air as it will heat up excessively as well.
The H50 CAN handle an overclocked/overvolted i7 but it is right on the edge as well. This is why they came out with the H70.