From what I have heard whilst visiting all of the grammars schools over the last week, ‘catchment’ is no longer relevant at all. It’s a yes or a no pass and then distance from the school.
The rest of your question has been very ably addressed by loobylou, but I should address this point.
Most of the Bucks grammars do indeed allocate well beyond their catchment area, but Dr Challoners Boys hardly ever does. They only did so in 2011 because of a nifty rule change. In other years a number of parents in Gerrards Cross, Denham and Iver, and in catchment for DCGS, would grit their teeth at your suggestion that "catchment" is no longer relevant!
Some other schools have occasionally been sufficiently popular to only allocate within catchment. You can see the allocation distances for the past ten years here: https://www.dropbox.com/s/kvy4xyyg3t0zr ... .xlsx?dl=0
Wherever the numbers are prefixed by "IA", that indicates a distance that was within catchment.
RGS is difficult to predict, but my hunch is that you should be OK from 10 miles away.