Good question. I haven't used Hi-tech B&W contrast filters per-se (actually, I didn't even know they made them!) but I do use their ND-grad filters and like them just fine. They fit without problem in my Cokin Z-Pro filter holder so I would think they would be fine in your Lee filter holder as well. For what its worth, I actually use the Lee gel filter set of B&W contrast filters for my B&W needs, but I might have chosen the Hi-tech resin versions had known about them originally.
The main thing I can say about the resin filters I have used is that they tend to be dust magnets! That experience is based on a very dry environment, so consider whether that applies or not. I do think they can scratch fairly easily though and treating them with the proverbial kid glove is advised. One suspicion I have long held is that the greatest risk of scratching occurs while sliding the filters in or out of your storage container (sleeve, holder, case, or whatever you use), which is worse in the field where they may be accumulating dust. The tight clear plastic sleeves the Hi-techs come in are really not ideal in this regard and I would recommend finding a different storage container if you can. I keep all my 4x4 and 4x6 filters in a Lee filter case now, the one with the oversized, velvety pockets. My sense is that the soft material and more generous pocket size of this holder makes a real difference in keeping down the proliferation of scratches on susceptible (resin or gel) filters.
