I think a "M5 competitor" would be a better phrase than "M5 killer". I think it will come down to personal preference. These cars aren't track cars so it doesn't matter if one is faster on a race track or not. In the real world of driving they will be very close.
2012 BMW 550 M-Sport
2015 Audi A7 Prestige
2014 Lexus GS350---Wife's car
2013 BMW 1600 GT---2011 Kawasaki Ninja 1000---2012 Ducati Panigale