You're best off talking to someone with a 3.6/R36 about exhaust options as I know people have experienced issues with drone in just removing the centre resonator. I believe you really need to change the centre resonator to a hotdog muffler and the rear mufflers to some sports mufflers to get a better sound without drone at cruising speed.

I'd be aiming for something like the Supersprint: .:R36 Supersprint Exhaust

To get better DSG farts you need to tune the DSG by adjusting the shift speed. There's other benefits in terms of adjusting shift points and letting loose the full torque of the 3.6.

You won't get massive power gains from tuning the 3.6 but if you're looking for decent deals there's a few specials being run around Xmas. I'd be talking to RD Technik who do package deals on DSG and ECU tuning current special is $1,275 (normally $1,499) for stage 1 ECU and stage 3 DSG tune.