Profabrics usually have Silnylon listed as Silicon Impregnated Nylon(which it is, its not coated), I haven't ever seen silnylon on Fabrics n stuff lists.
Silnylon Here.
Shelby.fi in finland and Extremtextil.com in germany have some really nice cutting edge fabrics with great descriptions to help you. They aren't cheap though.
When I made my last tarp, I used PU coated waterproof nylon which actually weighed less than the silnylon from Pro-fabrics. Silnylon for the same weight, generally has a higher tear strength though.
For a true lightweight job, Spinnaker fabric comes in much lower weights.
For a true mental ultralightweight tarp, Cuben fibre is the way to go but will cost almost double everything else and need joining with Cuben tape, not stitching.