40mm is a good size, I use a 70mm fan and it's too big.
Yeah Having it out of case is certainly helpful.

The buzzing from teh PWM's is very bad now at high load... It's hard to source where the buzzing is coming from exactly, but its definitely not the CPU fan and it appears to be the PWM's.

Maybe there is too much pressure? There definitely isn't too much heat.
It's not instable at all so tomorrow I'll remove the top plate, go easy on tightening it down and use next to no Thermal Paste.