The CLK430 and the CLK55 are very different cars. Not only is the 55's engine tuned by AMG, but so is the transmission (30% faster shifts) and all other major components. I've run against 3 different M5s at various times. Also, a friend has one, and we did repeated runs. In all cases, my CLK55 was slightly quicker, but when my friend executed "perfect" shifts on the M5 the performance was virtually identical. BTW, this is the same experience of another CLK55 owner I know.