I see the biggest problem in getting good FE is the temptation of the boost guage itself. It would be like trying to maintain a healthy marriage with a few UCLA cheerleaders living in your guest house. It could be done, but "deliver us from temptation" would be going through my head alot.
My wife has an unmodified '05 N.A. 2.5L Saabaru with a stick. She gets between 30 and 32 mpg with some minor hypermiling techniques. That's ~45% above what EPA says it's supposed to get (20/27, 22 combined). You could probably get 35 or more if you put some quality time into it.
My dad just bought one of those too! Yeah, the EPA mileage stinks, and Dad isn't too easy on the FE, he gets about 27 MPG average. With that in mind, I think you should easily get 30-35 with LoudRice by simply doing some minor improvements.
I've heard that CC's reduce noxious gases but actually increase greenhouse gases...what we need is some real ethanol like Brazil has instead of the corn scam ...or to drive less, which would be ideal.
As far as noise, its pretty tame under 3k RPMs and the sound is beautiful because of the boxer engine; I wouldn't want to make it quieter. I've had a number of cops follow me with the exhaust and the one who pulled me over did it for another reason without even mentioning the exhaust. I'm still surprised; the thing is huge, I can stick my arm in it down to the elbow
There is certainly an effect although I am not entirely sure what it is; Carbon Monoxide is not a greenhouse gas (but a poisonous one) and cats turn it into Carbon Dioxide which is. They also turn unburned fuel into Carbon Dioxide. I have read conflicting arguments of their effect on Nitrous Oxide which is a shorter-lived (I believe) but more potent GHG.