You can search and find a multitude of posts and some articles on this site about restoring sil coatings, usually on floors.
The problem is that the silicone needs to be diluted in order to coat well, and no one seems to have come up with anything outside a chemical factory that will really dissolve the silicone. So we try mineral spirits, camp stove fuel, and many other things. Haven't been able to find silicone sold in a highly diluted form.
The best I've seen on BPL is a mixture, referred to by some as a slurry, in which the silicone never really dissolves into the solvent. Add to that the fact that some of the solvents have some oil content that interferes with adhesion.
The result is eventual peeling, with some approaches lasting longer than others.
There are also silicone sprays, that are not so much waterproof as intended to provide a durable water resistant coating (DWR). I had an interesting experience with those. A much used silnylon fly from Wilderness Equipment was showing a lot of wear, so sprayed it with Atsko silicone. The result was not good. The fly fabric lost all its body and became flimsy and translucent; that is, you could see through it. Some months later, just for the heck of it, tried Kiwi CampDry silicone based spray(not the polymer based spray in the blue can), and all the body was restored to the fabric and it looked even better than when new.
So you have given me an idea. I have some "showerproof" polyester from one of the UK companies, that weighs a total of 1.3 ounces per sq. yd. It obviously has some kind of a DWR coating, but don't know if it is silicone or not. So will cut off some swatches and try both the silicone and polymer sprays from CampDry, Tectron and ScotchGaurd and see what happens. Don't have a Suter tester, but cruder tests will show if any one of the six swatches is markedly more water resistant than the others after spraying. Who knows, as with the WE fly, sometimes we can just stumble on to great results.
So what I am saying is that you may have to do some experimentation to try to arrive at a polyester that is nearer to the weight and water resistance of good quality silnylon.
P.S. Roger, I see you haven't changed your mind about Epic. But lots of folks have reported here of staying dry on long treks under tarps made of grades of Cuben fabric that Richard tested to around 500mm HH after not so much scrunching. The Epic treated Malibu polyester fabric goes to 1500mm HH, three times that. Assuming the Epic tent is not exposed to anything oily, like bug dope, and is well cared for and carefully stored to prevent mildew, might you be a bit less averse to its use? If you see something on the BBC news about some crazy yankee driving across the US with an Epic Malibu tent pitched on top of an Outback, it will be just me drying it out, and at the same time demonstrating that it is also just as wind resistant as a tunnel tent. The speed limit on I-80 in Nebraska is mostly 75 mph.