With Warm and Natural batting you can quilt lines up to 10" or so apart.
Instead of SID, why not do a serpentine stitch over those seams? It's a much more forgiving stitch, more fun to do, plus it gives a nice soft effect on geometric quilts.
My preference is for variegated Aurifil 2-ply 50wt thread, which is similar to Mettler 60wt. This is a slightly finer thread than the regular 3-ply 50wt that most people piece with.