Well..I do! I usually allow a smidgen more, but not much. 32g thereabouts.
Think it depends how strict you're being. You could try it, make sure you weigh it out and stick to your set limit, and see how it impacts on your weigh in result.
I think it's fibre vs. calories as long as you have the same amount of fibre within the same amount of calories - you'll be fine So if 32g of porridge has the same nutritional values as 28g of porridge and scan bran I can't see a problem