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The biggest change when going to a different muffled exhaust is going to be in the noise arena.
headers are great at making hp over a relatively narrow rpm range when
TDC compression cam overlap is increased
no muffler is installed
tapered post collector pipes are used, reverse cone added at the end spreads the torque out a bit
stock air cooled 911 cams from '74 up have little to no TDC overlap
stock mufflers are better at reducing noise than increasing flow, Mufflers are a balancing act between noise and flow, increase of one almost always reduces the other, this can be mitigated by a larger internal muffler volume and flow geometry.
Steve W, probably has more experience w/ different exhaust designs due to his chip tuning business. Check out the different setups on his site, as you will want one of his chips for a Motronic 911
I agree that w/o a lot of other internal mods a stock 3.2 exhaust, sans cat or w/ a high flow cat if necessary, and a well chosen muffler that you like sound wise, is the best way to go.
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