My data regarding the water resistance of silnylon was collected from a variety of sources. I should note, however, that silnylon is kind of an odd product about which not very much info is actually published, at least on the Internet.
One of the few companies that does make some specs available is Performance Textiles (http://perftex.com). Their "Soar Coat" product is pretty much a "standard" 30 / 40 denier silnylon. I ended up calling the company to discuss water resistance, since that particular spec is not listed on their website.
Another source, "Ayce" at thru-hiker.com, probably knows at least as much about silnylon as anyone else in the backpacking world and has confirmed the 1-2 PSI rating for "standard" silnylon. If you search his forum, you'll find the issue discussed in multiple threads.
As a further check, I built my own crude hydrostatic head testing rig that consisted mostly of a 10 foot section of PVC pipe to which I affixed silnylon samples (at the bottom when the pipe was oriented vertically). I'd slowly fill the pipe with water until the fabric sample began leaking, then would measure the height of the water column that produced the leakage. I found that the 1-2 PSI rating was about right for my silnylon samples when new, but also observed that the water resistance of silnylon can deteriorate after it's used for a while.
It's worth noting (as you already know, but others may not) that some newer 30d fabrics are now being treated with both silicone and polyurethane. The Sea to Summit Ultra Sil dry bags and the rain flys of lightweight tents like the MSR Hubba series are examples of applications using this dual-coated fabric.
The double coating adds weight, but improves water resistance to a degree (but not a lot in some cases if it's very thin). It also makes it possible to heat-tape the PU-coated side of the fabric—not possible with straight silnylon since it's so slippery. It further improves fire retardency enough that tents made from the fabric can be sold in those jurisdictions (seven states + Canada at last check) whose laws make most silnylon tents technically illegal because silnylon doesn't measure up to their tougher fire retardency standards. As most backpackers know, standard silnylon will burn if exposed directly to a flame.
So far, I've tested only a single sample of this dual-coated fabric (the new Ultra Sil dry bag) and was not very impressed. You can see the leakage problem here.