Jorge,
In theory you don't need the ND-CF in normal 36x24 mode the lens coverage is quite fine. But it's when you switch onto pano mode, that's when the lens design falls over and chokes.
Also forgive me if I'm wrong (I only was researching and thinking buying a XPAN last year), but the ND-CF filter is only really required for the 45mm lens.
But also try Doug's idea and try both. I have an old 150mm 5x7 lens I badger into use on my 8x10 with the add of a rear lens-mounted ND-CF filter.
Personally I've seen some nice B&W enlargments shot on a XPAN without the ND-CF, the dark edges on mono prints really draw you into the picture. However the photographer in question also pointed out to me that on colour prints it looks "yuck". Each to their own I guess.
Stu 🙂
PS. 150mm lens on 8x10 is similar to 21mm on 35mm.