I'm unfamiliar with Slingfin products, but I found this article that seemed authoritative, and has some good information about fabrics:

https://www.slingfin.com/blogs/the-beta/fabric-coatings-101-pu-vs-pe-vs-silicone

Also, this:

https://www.slingfin.com/pages/mzs-views

Interesting article. I've never heard of Slingfin before. Most of his point align with what I've heard from other knowledgeable people. Two things caught my eye:

1.
I've heard the opposite i.e. the silnylon tents will sag more in rain. I live in California and therefore don't do too much camping in the rain, but I haven't noticed much difference between my two tents (one silnylon and one PU) in rain performance though they are quite dissimilar tents and difficult to compare.

2.
This really surprises me as my sil tent is a Tarptent that is made in California and they shipped it to me in California. I wonder if Henry Shires is skirting the law or has an alternative interpretation of CPAI-84 that has not caught on industry wide.

I've always been mystified when people talk about silnylon sagging. My limited experience has been with the Ray-Way style tarp, which sags much less in the rain than PU-coated tents I've had.

In the CPAI84 specification, it mentions that in 1995 a group of tent manufacturers submitted a proposal to exempt lightweight backpacking and mountaineering tents, so maybe there's an exception in there somewhere.

That's good to know! I wonder if people's introduction to silnylon are similar to mine: the first silnylon tent I purchased also happens (not coincidentally) to be the first non-freestanding tent I purchased. As a result they never were too concerned about how much their old framed PU coated tent sagged, but if their new non-freestanding tent sags and they did a poor job pitching it then their tent might fall over.... and then they complain about sagging in silnylon.

I wonder how much of the sag, in either fabric, is attributable to the guy lines, which are usually nylon. It seems as though the fabric would have to wet out before it could absorb water and sag. Silnylon doesn't wet out, at least not that I can tell, and PU-coated nylon usually has a DWR, which of course, can wear off.

It would be an interesting experiment to treat the guylines with water repellent, or replace them with Dacron, and see if any sag is reduced. I'll let you rainy-day backpackers take care of that

Even in England the funny laws of california are apparent. I believe thermarests are sold with a health warning about birth defects !

I have to wonder just what they think a Thermarest is used for! (One guy I used to camp with named his. Note the “used to.”)

Well glenn thats why california have the warnings i suppose.

It's one of those multiple-use items backpackers are so fond of

Seriously, I guess Proposition 65 did some good, as some manufacturers stopped using potentially harmful chemicals in their products so they wouldn't have to label them (not so much in the backpacking world.)

At this point, I imagine the labels are mostly ignored on consumer goods.