Well, bit the bullet and decided to buy this beast instead :
AFOX Radeon HD 7850 (low profile)
It's the fastest low profile card available, as far as I know.
I have now installed it in a standard configuration and have been running some tests on it.
To keep the system power below 150W (my current power supply) I had to dial back the GPU, memory and power control settings in AMD Catalyst.
This will be remedied once my new 200W power brick arrives.
Even in this handicapped setup, it easily outperforms the Intel HD4000 graphics I've been using.
The fans are noisy, even when dialled down to minimum speed. Unacceptable for me so a silent solution is in the works. Heatpipes and 75x50x15mm copper bar is on order - the plan being to either mount the board directly onto the chassis heatsink (via a flexible PCI riser) or run heatpipes to the heatsink.
I had to remove the drive and the bracket for now to enable the card to fit.
Also, with the power connector located on the top side of the card, the top cover on the case does not close. I'll have to install a different power socket or solder power cables directly to the board.