Well... you asked for opinions, so here they are...
If you are going to supercharge your engines, then I would HIGHLY recommend that you get Whipples. I have one on my (single engine) 502 and it ROCKS!.
Whipple gives you the low end torque that you need for a decent holeshot. When you add 50% more horsepower, you will need to add about 4 more inches of prop. Without huge low end torque, you will barely get out of the hole.
However, that huge low end torque comes at a price. I have broken my stock forward gear in my Bravo 3 twice! I now have an IMCO "Heavy Duty" forward gear, and it has lasted (so far).
How long the XR drives will last with supercharged engines will greatly depend on your driving style. If you ease the throttles forward, they will last longer than if you "slam" the throttles forward (like I used to do).
In any case, a 31-foot Scarab with supercharged 502's should be a nice "sleeper"!