My thoughts are that 8.1" makes little sense for a screen size. At this point, after probably giving it waaaay too much thought, my standing opinion is that it should be 6" or less, or 10" or more. The in-between is really strange to me because it is still small enough that paper-sized PDF documents are small and hard to read if you don't have 20/20 vision, or you can't get whatever margin cropping voodoo to work right. I have a 10" screen sitting next to me here and I can barely tell the difference in size by way of visual estimation between a document on that screen and a real paper document. It's a good experience despite not being full 8.5"x11".
As for the smaller screens, I just wouldn't even consider them for page-sized document viewing and therefore my priorities would become portability and heft for long comfy recreational reading. As long as page turns are decently fast and fonts are well controlled, even a 4" screen can be comfortable to read.
So I think what you're going to get with this iRex is an inconveniently large device with an unremarkable increase in screen usability due to the extra 2 inches.