13-15 megs a second really will not make a difference. SATA II is 300MB/second, and you can feel it swap even on that. For reference:
SDCARD = 16?MB/Sec
IDE (ultra 100) = ~100MB/sec (never saw over 50 on bench marking...)
SATA-II SWAP = 300MB/sec
DDR200 = 1.6 GB/sec
DDR400 = 3.2 GB/sec
DDR2 400 = 3.2 GB/sec
DDR2 1066 = 8.53 GB/sec
Which is all dog slow compared to the actual cache.
So basically, unless you can get a card that is close to or greater than the speed of SATA II or even IDE (100MB/sec), i really do no see this as feasible.
That said, you are also limited to the BUS speed for the SDCARD, along with concurrency issues (ie, OS wants to read/write a regular file to the card, but the OS is also trying to swap to it, creating a race condition and slowing things down even more).