Yikes, I never tried this and I THINK the 1150 (certainly EU variants) was designed to have the full cladding only. If you tried fitting the 1100 fairing you'd be be left with naked fuel tank by the seat, so I think it's a cut job.
The full fairing is a pain in that there's more to disassemble before you can get behind it so the bike is harder to clean properly, remove fuel tank etc, but once buttoned up it makes a sort of external monocoque which must strengthen the bike, and it improves protection.
Try over on the Oracle RS forum - they're very helpful:
http://www.bikersoracle.com/rs/forum/index.php?, and they have quite a few US and Aussie members. The specs there are different and I think US 1150s might have been available with short fairings.
Cracking and underestimated bike though, it will ride through anything.
J