I have one Brooks Imperial B17 and one Standard B17 that I cut out myself. The one difference between them is that the self-cut one is made from thicker leather so it is slightly more rigid than the Imperial.
I don't know if the Imperials are made from thinner leather or this difference is just the luck of the draw.
If I had it to do over again, I'd have cut out my Standard B17 sooner. I love it.