The supercharger decreases gas mileage slightly under normal driving and a lot more under full throttle. Power needs fuel. Under normal driving my guess is 1-2 MPG. this shouldn't be a concern on this type of car. How much do you plan to drive anyway?
As far as service, the dealer can service anything that's not supercharger related. An advantage of the Comptech supercharger is that the engine management system is still OEM. So all your diagnostics and check engine light functions are the same. If there is a sensor malfunction, it reads like OEM. Many people are not aware that when you go to an aftermarket EMS, you lose part or all of that functionality. The AEM or whatever system is used can still provide diagnostics, but that's to an AEM tuner, not to a dealer. The dealer wants to plug in and read the OEM ecu. That is the same with the Comptech supercharger.