The W8 would be a little quicker in all straight line competitions over the RX8.
The RX8 is a superbly built car that does everything well except for acceleration. By design, it's a 1300cc that's only using 1/3 of the displacement at any given time. It operates like a 2 stroke, hence the poor fuel economy from not burning the fuel efficiently and needind to rev high to achieve adequate performance.
The W8 is incorporates complex German ingenuity. It's smooth, fast, has lots of torque but will not be able to negotiate the curves as well as the RX8, mainly due to it massive weight.
Both are fun cars, the W8 will burn a little more fuel but you're getting an instant return on your petrol money via the butt dyno.