All one needs to do is a Google search or search on Cloudy Nights for many reviews about this issue. I myself have tested hundreds of eyepieces from F/4.72 right up to F/6 using the eyepieces in their native form and with many different barlow lenses. My next purchase will more than likely be a Paracorr, which is needed IMHO with my 10" F/4.72 reflector for better edge correction on some of the eyepieces I have now.
The Vixen / Orion LVW eyepieces have been evaluated extensively down to F/4. Here is the translated site below. If you scroll down near the bottom, the LVW evaluations are there.
http://www.svenwienstein.de/HTML/hyp...h_version.html
Cheers,