I do not know much about the internal ballistic of different cases(i havent read the book but now its on my must read list,thanks), but i hardly think its only the
form of the case. Take a look at a 577/450, hardly a high pressure round, but probably the neck holds the pressure in a little longer and that way the powder will burn more even, and give a little more speed and acccuracy.

Any way, i think that backthrust or not, the pressure would be the same as the messured one.
And the brass in itself should not make much difference no mather how thick it is.