Hi. IMHO its the tips. Don't look right. Can't put my finger on it but don't complement the rear the way the M6 ones do. I would either investigate a set of very good tips from someone like Akrapovic or maybe Fabspeed or AWE and also go ahead and look at an exhaust too. Miestershaft makes one I believe that is controllable so you don't have to worry about sound/loudness as much so that may be an option. Quad tips don't look bad but the execution here is what probably has you scratching your head. Take a look at the new Alpina B6 from the rear...its quad tips look georgeous IMHO in the rear valance they did for the car.
http://www.roadandtrack.com/car-revi...-b6-gran-coupe
Since they all use the same rear diffuser basically that may be an option. However yes I agree that they do need to make a good Front Msport spoiler for the 650's. There are a few out there but you have to look for them.