I've used XENPAC w/X2 for uplink purposes and it worked out fine. Not sure about how SFP+. will work with XENPAC. Although, SFP+ is 10G capable, it has a few operational differences in comparison to the other two modules that may or may not hinder any performance.
Since the modules aren't cheap, I know you want reassurance. I would suggest you inquire with the vendor you plan on purchasing from (they are the experts and also perform testing). You may also want to inquire with Cisco sales reps (support).