1. If you live anywhere but Virginia, put it anywhere you want. It's legal in the other 49 states. Although, some states have some "gotcha's". Florida, for instance, you're not allowed to have anything on your windshield but a state issued registration sticker, or the likes thereof, or a toll device (Sunpass, Easypass, ect) because anything else "obstructs your view".
2.IMO, not at all. Talk with your dealer about honoring your warranty with aftermarket exhaust. I go to Fields and I have extreme modifications (supercharger, full catless exhaust, lowering springs) and they honored my full warranty. A lot of dealers have employees who are enthusiast like all of us, and are pretty lenient with M cars.