I have thought of this too and I have done it. I cut a blowhole right above the mobo and between the psu and cdroms. I placed the fan (92mm 50cfm Sunon exhausting) on the outside so it looks like it's hanging, but it's not really ugly.
I believe it helps a lot, especially if you have some fans for intake in the bottom front or opposite side-plate. I have 1x80 40cfm fan (bottom front) plus 2x92 50cfm fans (bottom left side-plate) for intake and 2x80 30 cfm fans (rear top) plus 1x92 50cfm fan (top right side-plate, the fan in question here) for exhaust and my case's temp is the same as my room's temp, the optimal condition.
What you could do to lower the *measured* cpu temps is to place a small fan like a 40x10 or a 60x10 on the outside of the case right below the cpu, blowing fresh air on the mobo. I don't believe a bigger fan would make a difference at that spot as it most probably won't be able to work at all, the mobo base-plate will be blocking all the air flow. If you are adventurous and you can measure well, you can try cutting the mobo base-plate AND the side-plate so you can freelly blow some air on the back of the mobo.
Cheers