I love red bell peppers. Is there a taste difference if I roast them before chopping and adding them to soup or beans that slow cook for 4-6 hours? Am I just cooking them before cooking them again or does roasting (under my broiler, then peeling) produce a unique taste that I wouldn't get from just simmering them for hours?
Roasted red peppers would add some subtle sweeter flavor and some smokiness depending how you roast it (over grill, wood ... )
But depending on the actual recipe it might just not make a big difference.
Also when roasting peppers like that, you will remove the outer skin which is hard to do with un-roasted peppers.
Having done some extensive testing on various chili recipes it is fair to say that roasted peppers do taste 'different' than unroasted. That difference is more noticeable if you also peel the peppers after roasting as the peel itself is something of a 'flavor barrier'. This page has a good tutorial on how to roast and peel red bell peppers, but the technique is good for any pepper I have tried. For my part, I prefer to open the peppers and clean out the spines and seeds and membrane before roasting and peeling. With hotter peppers this amplifies the flavor affect of the roast because it clears away most of the 'heat' (capsaicin) allow you to really taste the pepper and not the heat.