In post 97 of this thread I found out that the base of my heatsink is convex. However IC Diamond noted that my pressure is good as my IFX-14 has spring screws for tightning the heatsink and provide greater pressure. I must say my IFX-14 has quite a tricky mounting system and it is very easy to move the cooler if not careful.
http://forums.overclockers.co.uk/showpost.php?p=18506256&postcount=97
No amount of mounting pressure will eradicate a bad IHS mount. The IHS is soldered to the core on Q6600 onward iirc. Yours is not bad though, I wouldn't lose sleep over it. You might also want to consider lapping the IHS as the Core2 CPU's had notoriously uneven IHS's. This could even out your core temps a little if you are that concerned.
4 gram, seems silly not to invest in a few applications with results like these
You will get plenty of mounts out of a 4 gram tube despite having to use slightly more paste than lower viscosity TIM's.