Mark - I think you should chronograph the 100gr. of 2F and chronograph the 10gr./90gr.mix and see just how much difference there really is.
Of course, velocity isn't the main reason for special loads - accuracy is. If 100gr.of 2f is more accurate than the mix, I'd use that, or visa-vis.
Like Don, I've never seen any difference in ignition of the main charge, between 2f and 3F, except when comparing the 2 as pan powder itself. With proper sparks, the both go off - 3f priming merely ignites the barrel charge more quickly. 4f is faster yet, with Null B being the fastest of priming powders. All this has been proven by Rich.
Accuracy wise, we've found there is a difference with 2F being more accurate than 3F in every gun we've tested the two granulations in, from .50 to my 'little' .69. The slower and lower pressure spike of 2F also makes life easier for the patches.