exhaust pipe diameter for a mildly modified 340

I have read 2 1/4" diameter will support 400 hp. Any idea on how much power might be lost going to 2" diameter duals.
2 inch will be much lighter, leave more room and allow the use of quieter mufflers!
What's on there now? If I was to install a new system and 2 1/2" were an option I would go with that for a warmed over 340.