I think that installing a disk makes a lot of sense for your scenario, but to be honest, I have not tried it with any kind of load. The good news is that you can always try a simple approach and then reorganize if it does not work under load. I would hope that Rackspace is taken into account and tested for this exact scenario, it seems logical to me.
Cloud Site vs Cloud Files. , , / - . , , , , .
RackSpace - . - , , . - , API , .