Joe,

From all the replies you must understand that a tuned exhaust system does make a big difference.
Its effect is, of course, at mid-high to high RPM.

If you select a correct tuned silencer system (and adjust the low end needle properly), the idle and transition will remain great.

So the only major down side of such a system is cost (the weight penalty should be very slight).

If you select a Jett-Stream, or an Ultrathrust, the toll will be highest, the Tower/Hobbico muffler will be the the most cost effective. All other possibilities are somewhere in between.