Ok! So today I replaced the stock fans with 92mm units: Here is a pic of them installed:
The particular fan I used is
AVC 92mm x 25mm 4-Pin PWM Hydraulic Bearing Fan (DS09225R12HPFAF)
They peak at somewhere around 45cfm at fullspeed, the sound is noticeable but not unpleasant. They were easy to mount with zipties & small pieces of doublesided foam tape to stop vibrations.
I did not attempt to connect the fans to the oncard fan connector, as I was a bit worried they might draw too much current, but at some point I'll try this, just need to locate a mini-pwm connector adaptor. They are plugged into a fan controller for now.
On to testing: Idle temps are 4-6C lower, from ~45C to 39C now, a good sign. I then ran OCCT (zero errorrate clock settings) for 30min with stock fans and then with the new fans. Peak temps stock = 78C, new fans = 75C. Using a somewhat higher clockspeed setting that gives some OCCT errors, the number of reported errors was dramatically lower with the new fans, from stock many 100's to a couple dozen now.
So what conclusions to draw? Firstly, the stock fans are well-sized to the stock heatsink; even with dramatically higher airflow, load temps were only marginally better with the new fans. And this also demonstrates that the stock heatsink is the limiting factor to GPU temps; either watercooling or a larger aftermarket cooler will be needed to lower temps significantly.
Thirdly, and maybe most interestingly, I believe the lower OCCT error rate with the new fans is because they are providing more cooling to the memory and VR chips in back of the main heatsink. Even so, I felt the back of the card near the VR chips while OCCT was running, and it's still too hot to leave your finger for long; the stock heatsink for these chips is a simple strip of copper. Replacing it with real finned heatsinks that the better fans would provide increased airflow over could make a big difference, I'll try this at some point if I find a good deal on them.
So now I have a 4slot videocard that blocks all the other pci-e slots
Well, that's ok for now, and it's easily reversible. A fun experiment!