That is correct, with single or dual rank dimms, using all 3 dimms in the channel results in 800mhz.
With you wanting to use 8 dimms, you will need to populate as follows to get the 1333.
A1, A2, A4, A5 as well as B1, B2, B4. and B5. That is with the memory mode in the BIOS being set to Optimizer.