I was thinking about buying an SSD for my X200t for quite some time, but never wanted to replace the HD by an SSD. I always wanted SSD and HD to coexist, which is a bit problematic in a laptop that doesn't support mSATA. Furthermore, I've got integrated WWAN, so I wanted to keep that as well.
Finally, I came up with the following solution, which works perfectly fine, apart from hardware RFKill switch:
There are, however, two drawbacks: One must patch the BIOS for the Thinkpad to boot with the unauthorized card (did that myself) and Pin 20 on the card must be taped for wireless to function.
The system cannot boot from the CoreStore SSD. Instead, I've got GRUB on the HD, but system root on the SSD. So, the only part that is read from the HD are bootloader and kernel. I cannot (and don't want to) boot Windows from the SSD.
While upgrading, I also replaced the never correctly functioning Ericsson F3507g by a Huawei one - a half mini card. My first intention was to move the Huawei card to the half mini slot and keep my WiFi card, but the only port where the SIM card connections go to is the WWAN slot.