So a bit of an update here. I threw it all back together with new Gelid Extreme pads on the GPU, Thermal Grizzly Kryonaut on the GPU and CPU and honestly it didn't make much difference! Was still seeing 68-70c on the GPU during Heaven or 3dmark stress tests.
I had my top radiator fans in a "pull" configuration, so cool air was being sucked in from the front rad, but hot air pulled through the top rad. At 2am last night i drained the loop - CONSIDERABLY easier with my little air inlet at the top of the rad and drain port on the reservoir. Like - really easy. Nearly a litre simply poured out without any tilting.
Anyway, I changed the top fans to be an intake too. So ALL intake, bar the 1x140mm rear exhaust fan. Well wouldn't you know, my temps have plummeted. Now a maximum of 61c on the GPU no matter what I do (with case completely closed and fans on max). In real world gaming it seems to hover around 57-59c. So from the previous setup before teardown, I've dropped the GPU temp from 70-72c while gaming to under 60c. Very happy with that result, and means a much quieter computer.
Also, the CPU temp at 90c was largely due to PBO2 settings in BIOS. With the bios reset to defaults, the CPU would not get hotter than 65c during Cinebench R23. With PBO2 turned on it was 72c but only boosting to ~4100mhz across all cores. With other PBO2 settings I can boost to ~4550mhz on all cores, but it basically boosts until the BIOS imposed thermal limit of 90c.
So... a mixed result of the teardown. In hindsight I should have chaned the fan orientation first and it probably would have given me 80% of the gains I've seen in doing everything.