Need to decide today between buying a new 545i Sport or a used Audi RS6. The RS6 is famous for being easily upgradable; from what I read you can get an EPROM flash for $900 to bump the horsepower into the mid-500s. What I want to know is if I want more power out of my Bimmer a couple years down the line will there be affordable aftermarket options to give me that power, or are the Bimmer engines already so well-tuned that you basically need to spend $20k more for any kind of major HP upgrade? (Yes, I am a clueless Lexus owner, and no, an M5 is not an option because I drive automatic).