Merlin,
I was reading down and noticed you rectified your confusion between the UO orthos and the older 3 element designs like Ramsdens and Kellners
I own a full set of the newer UO HD orthos, in addition to a set of Pentax XW's. I have a 10"/F5 newt and the UO HD orthos give excellent views to the Edge of Field in my F5 scope, which is as fast as I have used them in. I think they may struggle at anything faster than F5.
I really have no need to keep the UO HD orthos as the Pentax XW's cover all bases, but I just like them because they are so small, light and easy to handle and give superb image quality. They also make great "loaners" if someone needs something at a specific focal length at a star party in a hurry.
CS-John B