I haven't used the ones by Mission, but I am a huge fan/use or the low fat/low carb tortillas. I use the ones by LaTortilla factory. The ingredients are primarily oat fiber, whole wheat flour and soy flour. This is the nutritional value for the small tortillas:
CAL: 80
FAT: 2g
PROTEIN: 5g
CARBS: 11g
FIBER: 8g
As you note, they taste great. For me, they make it easy to meet my target for fiber (40g+ per day). Invaluable. Sorry I can't be much help on the "modified wheat starch" question.