I have talked with a naturopathic doctor about diet and SCLS, and her recommendation was to eat as though I were diabetic - very low sugar, grain, carbohydrate consumption. In the holistic health community there is a growing perspective that auto-immune issues are fundamentally about inflammation, and that the Paleo diet approach is key to getting that under control.
IViG works really well for me, so I can't say whether or not diet makes a difference, but I do know people for whom this diet makes a huge difference in chronic pain and in recovering from Lyme, which seems to mimic an auto-immune condition and responds to being treated as such (again, from the holistic perspective).
If I ever lose insurance and have to go off IViG, I will rely on diet, homeopathic remedies, herbs, and nutrition. What we put in our bodies makes a tremendous difference - we ARE what we eat, so it only makes sense to eat whole, natural foods.
Best of luck to you :)