Thanks, I'm glad to hear my idea wasn't crazy as I don't have a lot of experience stepping outside of basic usages shown in datasheets.
Crossroads, the micro I'm using is a 168MHz STM32 ARM so the 21MHz SPI wasn't an error. I'm just more comfortable with this forum and figured my question was general enough for here. Sorry to disappoint if you were hoping I had some spectacular 8bit AVR over clocking skills. 