A couple more things/opinions
1. Through testing with my xb2003, a sportmaster had much more inherent lift compared to my JC's modded 200 case. I could maintain the necessary lift with more motor height versus the JC's 200 case. Only problem was that my JC's case had the gear ratio I needed and I never spent the money to change the sportmaster gears. I have a feeling the sporty may have been quicker (not necessarily faster) on that big boat, but I don't have the scientific data to back that up.
2. You mentioned tandem seating, so I'm assuming you'll have a passenger quite often. I think this plays right into the strenghts of the sporty mentioned above. The xr doesn't require the lift that the xb did of course,,,,but a couple hundered pounds in the passenger seat will change your setup quite a bit. I would think the sporty would handle the weight change a little better.
3. The stock sporty has an excessive amount of torque tab built into it (based on the testing I've been around). If you experience some ill handling,,,the first thing I would do is test less tab. Pretty easy to do, starting with a stock sporty.
9 inches of setback with a drag should work just fine.
