I recently changed from Lee dies to RCBS X-Die set for .223. I wasn't fond of the Lee dies, and I'm even less fond of trimming rifle cases.
I am just a few steps ahead of you in that I have done the initial trim on a dedicated batch of 500 cases. I have marked the heads so that I can keep them segregated for subsequent loadings. I'll see if the theory works in practice. Otherwise, the die will just be used as a traditional sizer.
My understanding of the benefits of the X-Die is not so much about extending case life, but in containing the growth of the length of cases so that trimming is not needed as frequently.
M1As are murder on cases. Best gains will be in setting the shoulder to the right length. Either way, I am not crazy about loading more than 5 time in M1A brass.