Abe - Good question. Also comes up from time-to-time on most online backpacking forums.
1. I've not tried McNett's silicon spray, but have tested similar products such as Kiwi's Camp Dry 13% silicone spray (see below).
Although these products contain silicone, I've found that the spray does not bond (at all) with silnylon fabric. Not sure exactly why, but these products simply rub off after being applied, even if allowed to set up overnight. Probably has to do with the fact that the existing silicone impregnation prevents the spray from being absorbed into the fabric's fibers. These sprays do seem to work fairly well on untreated materials.
2. Polyurethane-based treatments will likewise not bond with standard silnylon and will simply rub off if applied.
It should be noted, however, that some lightweight shelters now use a hybrid fabric that's treated first with polyurethane (usually on the inside surface), then impregnated with silicone. The PU coating helps to improve both the fabric's water and its fire resistance. These fabrics can usually be identified by feel, since the silicone side will be very slick and the PU side much less so.
McNett's Tent Sure floor sealant would probably work fine on the PU treated side of these hybrid fabrics.
Hope this helps...