As I never do long rides loaded down on my ST4 (that's what BMW-riding buddies are for, pack-muling!) I can't say realistically what kind of effect loaded bags have on tire wear but I can tell you that even with my aggressive riding (twisties included) I got just under 8K mi out of my Sports. That was pushing it. And I keep the pressure relatively soft.

I think it may be Metz M6's or something with a 6 in the name; my BMW buddy got a set earlier in the year based on high reccomndtns.

FWIW I have been hearing great things about Pilot Powers (which are actually the Sports' successors - Sports are no longer being produced from what I've learned) though they may not take you past 6K.