Not sure I agree with mixed mounts as it will put more strain on the solid mounts than the stock ones. Try this on a front engine rear drive car and you break stuff (ask me how I know LOL). Just from a stress standpoint I'd use the same type of mount all the way around, but hey that's just one nutcases opinion hehe
FWIW I've run solid mounts on plenty of big cammed dragcars the vibration isn't any worse than the sloppy stock mounts that let the engine bounce around on mild cammed motors it's more noticeable but not bad, I only mention this because the type of motors I'm refering to are nowhere near as smooth as a 911 motor. One thing to think about also is that 911's are splitcase motors and solid mounts effectively make the motor a stressed part of the chassis, not sure if that's such a good thing for a street car with a motor prone to oil leaks. FWIW