I ran our 300E without its rear muffler for a few days, then replaced both mufflers with OE equiv (Eberspacher). other than the noise, there was no noticable performance difference. louder is NOT faster, its just louder. and more annoying to your passengers and neighbors.
The stock exhaust on these things, with the dual 3:1 headers into twin cats, dual pipes into the first muffler, then one larger pipe into the 2nd muffler, breathes just fine. unless you're slapping a stage III turbo kit onto one of these and doubling your CFM airflow (and halving your gas mileage), you won't see any gain from bigger pipes or louder mufflers.