I don't remember what the final 'definitive' answer was, but I think that the 2.25 was considered sufficient for what there was. I would probably be inclined to go 2.5 to the cat and 2.25 from cat back, but I'm talking off way old memory here, so I'm sure someone else will hop in with better info.

As for tips, some will change tone and sound somewhat, but mostly they're glamour goodies, totally intended for looks.

James