Hello again John,
Here I am again with some more information. I just communicated with Mark Segal, who is of course a very in depth contributor to this forum. He had written an excellent review of the P800 in July, 2015 and I knew he was using this printer, so it seemed to be the place to start. He passed along some valuable info concerning the "Paper Size Check" setting I have referred to, and how to find it on the P800. As I thought, it is indeed embedded in the control panel of the printer and this is the path. On the control panel display go to the "Set-Up" menu. From there "System Administration > Printer Settings" and scroll down to "Paper Size Check". He agreed that the default setting should be "ON", but if not then definitely check the "ON" box. As I have mentioned, I do not know if this will solve your issue. I am only offering this route based on my own experience, when, for whatever reason I had turned that setting OFF and then forgot to turn it back ON before running a roll paper print. If I don't catch it in time to pause and delete that job it will indeed waste about the same amount of paper you have described. Of course I then flatten and cut that "wasted" paper and use the 8x11.5 sheets for testing purposes.
Good luck with this John, and thanks again Mark.
Gary