Here's the back of the case - Using countersunk screws inside means I need to have nuts on this side.
IC cooling are kind enough to send out free samples of their Thermal interface material if you agree to test it against your previous paste and send them the results.
So after sorting out the kink in the watercooling loop and testing my previous paste, MX-3, using Intel Burn Test, and using InterBurnTest and MSI Kombustor/Furmark simultaneously (to produce as much heat as possible - around 550W!) I swapped out the TIM and remounted the blocks.
Those rivet nuts aren't bad - bit big though, and I'd like to still be able to take the panels off in the future should I need to.