The RX-8 motor is lighter, but there are many drawbacks to Wankel rotary engines. First, it generates roughly half as much torque as its horsepower output. Second, it gulps gas faster than a V8. Third, as Mazda is the only company producing parts for it, maintenance is more expensive than other cars. And fourth, its lifespan is shorter than a conventional engine. Because of at least two of these four reasons, many RX-7 owners have swapped their Wankels with Chevy LS1 (some have even used the newer LS2) V8 engines.
As for the Mazdaspeed engine, I'd also like to know how it would handle on a RWD setup.