selfraising flour vs plain I use 1 teaspoon baking powder per 150g (or so) flour instead. never use self raising flour as I bake a lot with yeast. for the banana bread, what you plan to do sounds fine.
So, I used about 25g bran and 150g plain white flour instead of 175g of wholemeal flour. The loaf took longer to cook than usual (it's a tried and tested recipe!) and still seems quite moist and almost claggy right in the middle. I did read that wholemeal flour needs more moisture so that could be why....probably not helped by the fact I only had large eggs, rather than medium! If you can reduce the amount of wet ingredients in your cake, I'd definitely recommend that. Anyway, it's fine, the loaf is probably not the best that I've ever made but it'll be edible.