I've put thousands of miles on our SMB while carrying a 280lb bike on my moto jack rack. Key difference is that we don't have the cargo box on the back. As a result, I don't need to use a receiver extension for the moto jack rack due to clearance issues. The bike clears the rear spare tire by a few inches. But there is no rubbing on the spare tire and the bike is carried both higher and closer to the rear doors. It is, however, a bit of a PITA to get the bike up onto the MJR. I've got to use some creative body english to get it up about 6-8 inches onto the MJR platform.
The first couple times I carried the bike on the MJR behind the SMB I DID use a receiver extension because it was easier to load the bike. Big mistake. With the extension, there was no way to stabilize the play inside the hitch receiver. As a result, the MJR and bike bounced around in a VERY worrisome way on even small road bumps. That was very stressful.
If you can find a way to properly stabilize the free-play in the hitch receiver, you should be OK. IMHO, that is the key variable more so than how far back the bike is carried. My MJR has a built in stabilizer and it works very well. I don't notice any significant difference in handling when driving with a bike on the back.
2006 SMB 4x4, EB-51, 6.0psd