Hello,
The XF on one report I read didn't rate so well, that's one report from one user though.
I have the Vixen LV-Zoom 8-24 zoom ($199 new Oz) and its very good on planets in my 8" F6 dob. Not sure if you have Russian talks on your reading list, a reasonable list of EP's to help compare the numbers and stated for F4 and F10 scopes.
http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483
Steve