Should work ok as long as you used just enough compound to eliminate air pockets. Is that Arctic MX-4 or Arctic Silver 5 compound ? Arctic MX-4 is non conductive/non capacitive. Silver 5 is non conductive but is capacitive and should not be used anywhere it can get on traces, pads, etc. I like the Ceramic or MX-4 for places that has pads and traces nearby like in your situation.

The cache chip's plated octagon shaped top is made of copper and is not needed for shim, the bare die does need copper shim to get even height with the cache IC. See if you can do that way and use larger heatsink to clamp on and work?

Cheers,

Wait a minute, the octagone shaped chip is the cache? Anyway i need to buy now fine sandpaper, because the copper pads have sides and i have to make them perfectly even.
Thanks for the paste warning, i will be careful with!