In short, I'm not satisfied with the disk performance using LVM's full disk encryption on 12.10. Running a Samsung 840 Pro in a modern Thinkpad, I'm getting <6MB/s and under 1500 IOPS.

That's unacceptable.

Without reinstalling from scratch, is there a good way to disbale the LVM encryption and try the hardware encryption my SSD offers? Or do I just need to bite the bullet, wipe, and reinstall?

Yes, if I'd been thinking, I would have just used the hardware option to begin with. But suffice it to say that I wasn't thinking when I did the install. I'd actually forgotten that my SSD had that feature.