Extrem Textil has a 20D silnylon that it rates at 2000mm HH – Comes in 3 colors, and weighs 1.16 osy (my scale). Overseas shipping by ET has improved greatly and it now takes credit cards without a PayPal transfer. Noticed that material has an electrostatic quality that attracts small sand and dust particles – but that is common for silnylons.
S to S Escapist tarps are reputed to include a silnylon coat, but exhibit qualities of PU, including less e-static. Both sides of fabric adhere to PU, but haven't tried any serious bonding with it yet. Fabric is 1.06 osy (my scale), but rated only to 1200mm HH and only comes in lyme. Very expensive way to purchase fabric compared to the ET stuff.
Terra-Nova Tarp Ones are rated at 3000mm, come only in dark green, and weigh ~1.15 osy (my scale). Coating is all sil, but doesn't appear to attract dust. They are 100% sewn, not bonded, so entire tarp is usable fabric. Again, precious way to buy fabric.
Don't have independent HH tests for any of these. Maybe will shoot an email off to Richard this Spring.