| Different for everyone, but you can see a slight difference between your BGs 2-3 hours later with the same bolus on the same amt of carbs of Rice Crispies and Corn Flakes.
With Corn Flakes you will be slightly higher. I've been told it's something natural that occurs with corn. I usually can get away with just bolusing a bit more--no need for an extended bolus.
As for cornbread--you'll find lots of added sugar--I always tend to bolus more than I need for cornbread. After the first bite you can usually tell if you're going to need more insulin (it tastes really good!). | 
