I'm picking up my '06 Premium GT M5 this weekend. I'm going to have about $700 to spend on mods. I want to get an nice sounding exhaust system and tint first thing (it's a Black/red car so tint is a must). I want to go with the Mac exhaust but I don't know whether to get the axle back or the cat back. I plan on adding a Mac prochamber later on down the road. If I just get the axle back now will I still be able to get the over-axle pipes on down the road? It's my assumption that if I want to have a true 2.5" system from the manifolds back I'm going to have to do the complete catback with the Mac prochamber. Is this correct?