Actually stage 6 is out for the 550
. I had a 2012 550xi stage 4 and it was a fun car for what it was...especially all the torque you can just lay down. Now I have a F13 M6 and I can tell you that a 550 stage 4 does not compare to the motor, tuning and overall feel of the M5/M6. It was fun building the car and I had a lot of fun with it but by no means does it compare.