I have almost the very machine you are thinking of. Yes, you need the RAID card to use SAS drives. And yes, even without it, it is fast.
Keep one thing in mind. A 15K RPM drive is likely to generate quite a bit more noise and heat (hence more fan noise) than a 7200 RPM drive. Is it really worth it?
Personally, I am attacking the "need for speed" another way. I am presently in the process of adding a second hard drive to my beast. I have discovered that Seagate now has a new member of the same drive family that Apple uses for these machines that is 34% faster than the one that the machine comes with AND has a 32 MB cache, not a 16 MB cache, all while still spinning at 7200 RPM. It is only $129. I couldn't resist. It should be arriving today.
I will use SuperDuper or CarbonCopyCloner to clone my current drive to the new one and then boot from the new one. Hopefully, I will see speed increases across the board.
This might be an option for you too? Also, if you are using Photoshop, note that it runs more efficiently with two physical drives (not two partitions, but two physically different drives) than one. Hence, the above should improve Photoshop's overall performance as well.