well, back in the day we were hanging HUGE AX7's and HEAVY SLK800's off of the much much smaller AXP die, and those didnt even use the mounting holes (the AX7 definately only clipped onto the socket.
http://slo-tech.com/slike/293.jpg <who remembers this bad-boy combo? (review
http://www.frostytech.com/articleview.cfm?articleID=1274 )
if its a worry - carefully make a shim out of layers of electrical tape. this is what ive had todo to get good contact with my decapped chip and the RBX waterblock (the koolance block used to fit fine without a shim!)
uneven contact could definately hold back an OC. i used to be able to run 2920mhz @ 1.5v @ ~69/~80C load. lapping took it down to 1.475V @ ~68/~75c decapping got ~40/~43c @ 1.425V (mine was only just in contact with a thick layer of goup under the IHS)
now its decapped i can also run at 3100mhz @ 1.65v with ~50/~55c load...