Rear Spoiler concensus

In line or slightly above the roofline is the go. Mounting to the chassis works alit better than just thebboit too. If you can only mount to the boot replace the rubber stoppers with something that holds it solid (the japanese generally use latches) there is alot more you could do, but this works.

If anything it would destroy the little laminar airflow left and create more turbulence and drag than down force. The best thing you could do to the roof is get the airflow to pull back down, a hatch over the engine compartment is the best solution for this, it also means you can sit a rear wing much lower down and would also reduce over all drag significantly. 
 
There are other alternatives to keeping the air flowing through this area but a hatch is relatively easy to make, provides better cooling for the engine bay, gives you a fantastic slim location for the intercooler and would increase top speed. The biggest problems would be managing the engine bay airflow, obviously some redesigning here would also be in order to keep the two areas of high pressure from mixing. I'd probably look at vortex generators on the roofline a duct for the cooler then tapering the airflow from the cooler so that it encourages airflow back out the top behind the intercooler. I'd imagine a rear defuser wouldn't go astray either, the airflow from the engine bay and exhaust must be horrible. 

Stuka,
 
Apparently, 90kms/ph is the threshold of wind tolerance. Tarmac rally speeds are open so 90k is slow.