Not positive about the construction of the BA Sarvis line of tents since I think they've been discontinued (at least they're no longer on the BA website). However, BA does use the silicone/PU combo for most of its other tents, so I'd guess that yours use the same construction.
Most major shelter manufacturers are now using this silicone/PU combo fabric for several reasons. It's more water-resistant that standard silnylon, the PU coating allows the seams to be taped (not possible with regular silnylon), the PU coating makes the inside surface a lot less slippery than standard silnylon, and the PU coating satisfies the fire-retardancy laws in all US states and Canada (standard silnylon does not).
The only downside is that the silicone/PU combo fabrics are slightly heavier than standard silnylon, but not by all that much (~1.5 to 1.6 oz/sq yd for the combos versus ~1.3 oz/sq yd for standard 30/40D silnylon) .
Bottom line: I wouldn't bother applying the treatment described in the article unless you really wanted to beef up the water resistance (probably unnecessary). And even then, you'd only want to apply it to the non-PU coated side, since the treatment would not stick to the PU coating.
Hope this helps...