depending on the type of bend the muffler shop does, and how many piece of equipment (muffler, resonators etc) you put in, you will only need 2 - 2 1/4" exhaust. the more "free flowing" the set up, then step down to 2". The only way you would possible need more than 2 1/4" is if you have turbo..

this will give you the most lower rpm and be more than enough for the top end flow. Even the stock exhaust mid section is acceptable with a good muffler.

the "powerband" question simplified, "ideally" you want to allow the cams to be the only thing that restricts air flow. so opening up the restrictions elsewhere in the system (intake and exhaust) eliminates premature starvation outside the engine block. thats really simplified.. to almost sound wrong.