good question. in the one instance where I have had such a seam on a pack (extension collar to main body), i used silicone on the silnylon side and seam grip on the xpac side. good results so far.
on the flip side, i can say definitively that silicone sealer will not stick well to a PU coated fabric (from firsthand experience before I knew you needed the right sealer for the right fabric). I've never tried seam grip on silnylon though, so I don't know for sure that it won't work, but I doubt it would.
hope that helps. cheers,