Having gone from the stock system to the Eisenmann System, they are not too different BUT both systems have a bit of drone on the highway and the Eisenmann system is a bit louder but does pop nicely on downshifts and trailing throttle [not excessively], and performance was improved a bit. I did not record any difference in mileage though. Following the Eisenmann installation, the addition of a set of 313 wheels, H & R Sway Bars and Springs, I had a friend up at the Ring tune both the engine and suspension and he was able to get 285HP out of the motor with no loss in mileage as opposed to about 270 out of the cars with the stock system. I do not find the drone objectionable but i think my morning and evening commute is a bit faster than yours [long stretch on the way to work with no speed limit], and I do bi-weekly trips up to the Ring [2 hours] and am not bothered.
2007 E81 - 120d Limited Sport Ed
2009 E89 - sDrive3.0