The issue isn't likely the data transfer abilities from your drives. That's actually not the weak point for this. (Obviously this is not trivial either but it sounds like you have this part well covered.)
Anything over 100 tracks at high def sample rates and Reaper gets heavy audio threads and has a little trouble getting out of the way of itself. (Or so this is starting to appear to me. Other DAW's have this issue as well BTW.)
For playback (or even recording where real-time live sound monitoring is not needed), increase the disc buffer size to 1024 or even 2048 to give Reaper some room to work. Preferences/Audio/Device -> Request block size. Check the box and enter the number.
See this post for more:
http://forum.cockos.com/showpost.php...5&postcount=40