[grumpysnapper]

Decided to give the setup a proper check after a week of hard use, so took off the rear bumper.
Its working really well, temps appreciably lower in areas/uses that used to concern me, and cool down after a hard run is noticeably quicker.
It's venting very well, and appears to be drawing a reasonable air flow through when the car is at speed, and the fans are not operating... the negative pressure at the rear might actually be working to some degree.
So far, a success. (touch wood!)