If you are using low thread tension, and stretch the fabric as you sew, then when you stop stretching, the fabric will return to its unstetched condition, and the stitches will have loose loops.
What we need is a high quality light nylon thread that will expand and contract with the fabric when the temp. changes. All the ones I have found were too heavy to do this effectively. Theoretically, as nylon is stronger than polyester, it should be possible to have finer nylon thread that is also stronger than the polyester.
In the meantime, I think all we can do with silnylon is to sew with low thread tension, as low as you can go without creating the loose loops, and sew with the fabric at normal tension. If the fabric is puckering after sewing, the thread tension is too high.
Haven't tried the zig-zag stitch, although the sailmakers seem to feel it is worth doing. But they are using basting tape and just a couple layers of fabric. If the seam is lap felled, with reinforcements and maybe some pole sleeves or netting as well, that adds up to many layers of material at the seam vs just one layer outside the seam. Not sure that a zig-zag stitch at the seam is going to help that much with such a thick seam. But would be interested to hear otherwise.
Design also helps. If the tension can be kept uniform over the canopy surface, as with a hooped tunnel tent like yours or the Warmlite, for example; then it is easier to reduce the sag by tightening the tension on the canopy.
I think you and some other folks have posted about using shock cord or other elastic material on the guylines with some success. But it won't work if the design is such that the seams, being less elastic than the canopy surface, prevent the guys from pulling the canopy taut.
Would love to know what experiences owners of the silnylon mids have had with this problem, with the triangular and catenary shaped panels with seams that do not run parallel to the grain of the fabric.
Wish I knew more about this subject and could deal with the sag issue better.