Now you all have got me more confused than ever about the terminology, BUT, am still clear about the substance.
Looks like my text and diagram failed to communicate. Not the first time.
There is no argument about differing strengths. With any of these seams, there is a line of stitching, therefore a line of perforations, that are pulled on when the the fabric is tensioned along the seam. As with force 10 winds blowing at, and sometimes through, a tent, often in bursts that are positively scary. With poor fabric, the line of perforations will operate just as they do on a postage stamp. With good quality nylon or polyester, the perforation holes will elongate a little in the direction of where the tension comes, but the fabric will not fail.
With a flat felled seam, to use the term as defined by Judy here and on the companion thread, when those holes elongate, there will be four layers of fabric penetrated at each hole. And there will be sealer all through the needle holes penetrating those four layers of fabric. Even if the seam is sewn less than perfectly, there will be three layers of fabric penetrated.
But with a faux felled seam, often seen on cheaper tents, it is a different story. The diagram I placed in the above post is of a faux felled seam. Please excuse the term 'faux' if you don't like it. It is the most common way seen in the literature that's used to distinguish it from a flat felled seam. And I think 'faux' is a fair term, because it looks like a flat felled seam, but isn't. For that reason, I can't respond to Lawson's picture. To tell the difference between the real and faux, it is necessary to eyeball it very closely, or touch both sides of the seam with thumb and forefinger.
In the diagram I posted above, the one layer of fabric leaving the stitch line on the right has a row of stitch holes that has absolutely nothing under them except maybe some sealer if the article was also sealed on the inside. Nylon is a highly elastic material. Polyester no where near as much, but try and find good HH coated polyester in the one ounce range. So when the silnylon is tensioned away from the seam, those stitch holes will elongate, and the only thing in that long line of stitch holes between you and wind driven rain will be a miniscule amount of sealer.
I have indeed experienced the result of this when the sealer ages a bit - I got very wet and miserable. Sealing over the aged sealer doesn't work well either. Tried that. Maybe not quite so bad with silicone rather than PU, but still a problem. A problem not worth bothering with if it can be totally avoided with a flat felled seam that puts layers of fabric at all the stitch holes.
Hope everyone groks what a flat felled seam is - just two folded edges interlocked and sewn a mm or so inside the folds with two lines of stitching. Can do a diagram if needed.
With heavier fabrics and smaller items, all of the above may not be an issue, as has been suggested. In packs, simple french seams can be used, sealed, and the binder strip sewn over the seam. That's what Eureka and others do on tents. But there is no reason not to do it on packs in areas where an overlapping seam just isn't worth the trouble. With stuff sacks, though, a more water resistant seam may be worth the trouble. It is for me, having seen what happens once water gets into the pack after hiking in the open in the driving rain all day.
But a word of caution - the Dimension-Polyant fabrics behave more like Cuben than nylon when stitched; in that the stitch holes don't have the 'self-sealing' qualities of nylon. Sure, the material is 'waterproof' before sewn; but once lines of stitch holes are added at the seams ... . That's one of the reasons I stuck with nylon, especially when there are lighter sil-coated nylon balloon cloths and other products out there now to experiment with.
I apologize if the above sounds pendantic, which it probably does. Sorry. Maybe some good will come of it, though.