I'm not refuting your GI point but the combination of foods does have an impact on how they are digested. E.g. want to fatten a cow? Give it plenty of water with its food.
Other combinations such as wholemeal bread & fats are better than fats without wholemeal.
Denatured flour will re-absorb vitamins & minerals from the other foods on the plate.
I'm not advocating the Shang-ri-la but http://www.sethroberts.net/ has some good reading on the subject, particularly "What Makes Food Fattening? A Pavlovian Theory of Weight Control" - http://www.sethroberts.net/about/whatmakesfoodfattening.pdf