Scott,
My tank is about two feet below the engine and I have no problems with fuel delivery. I may be close to the lift limits of the pump though. I can see where your tank might be a special problem. Does your return line have the flow restrictor installed? If not, you may not be reaching the required pressure at the carbs. If you are very close to the lift capacity of the pump, the return line might cause just enough pressure loss to lower the pressure you need at the carb. You might try temporarily clamping the return line shut just to see if the pressure comes up enough to cure the problem. Many engines are running with no return lines at all. Unless you are having a vapor lock or overpressure problem, you can operate without it. If you already have the proper pressure at the carbs (2.2 to 5.8 psi if I remember correctly) then the problem is not the fuel pump.
With a three foot head for the pump to overcome, you may be stuck with using the electric pump. I have a facet electric pump on a continental 0-200 that runs all the time the engine is running and has done so reliably for years. It pumps fuel from the low wing tanks to a header tank which gravity feeds the engine, which has no mechanical pump.
Another way you might go is with a Billet piston pump made especially for the Rotax (but not made by rotax). It is a piston pump with one moving part, no diaphragm, and bolts right on where your present pump mounts. It also comes with an adjustable pressure regulator.
With a fuel tank that far below the engine, I can see why you might need to do some tinkering with the pumps.
