I'll probably get stabbed for saying this, but separation isn't everything.
I find Dolby Pro Logic II Music, with Panorama set to on, to be fabulous.
So fabulous, it puts about half of all true surround mixes to shame.
Just speculation, but I would bet that if there were any compromise in F to B separation, it is a) very slight and b) done for a good reason. I'd guess that reason might be to avoid pumping effects or other artifacts. Dolby was the master of the compromise, and while many here think they are the Audio Antichrist, I believe most of the design choices they made over the years to be good ones.
I had an Audionic Space and Image Composer in 1980, a fairly late-generation model. Loved the separation, hated -- absolutely HATED -- the hissy sound quality. Listed S/N was 70 dB -- pitiful, even for 1980. If I still owned one, there is no chance whatsoever that I would use it for creating surround from stereo in 2007.
There is quite a bit of question as to exactly how much F to B separation is even subjectively discernible. Although certainly many here will probably do far better than the average listener on such a test, deciding to use an expensive, clunky, thirty year old technology over an affordable, practical modern system to acheive maybe a dB or two of questionable additional separation is, to me, hair-splitting of the highest OCD order. I don't recommend it.
