Spider silk (and presumably DS) require considerable force to stretch, so it would not be a good substitute for Spandex.
But the larger question might be restated: what percentage of DS would alter the appearance or performance of a fabric in a way that buyers will pay for? I’m not talking about novelty items like Bolt’s tie.
Spider and Goldwyn put out a hoodie that had a paragraph silkscreened on the front explaining the clothing was made with a blended fabric. If you have to print a paragraph on a hoodie to explain how special it is, fabric-wise, that’s still a novelty item.
One of the coats included 60% brewed protein in the outer shell, while another article only had 10% brewed protein.
My guess is that it takes at minimum of 10% DS before a textile seems different enough from something made with mundane silk to appeal to buyers. This is only a guess, but I think it will be a long time before one sees a suit with 2% DS in it.
You probably know that KBLB seems committed to cotton/DS blends for their initial product release. All of us hope the day is near when we can see Spydasilk products for sale online. But I don’t expect it soon.