It was my understanding that they were electronic limited to 157, but mine only does 154. Radar detector GPS speed said ~155. With the 10 Speed and 3.55's, I was at like 6800rpm in 7th so I didn't have much more speed left anyway.
I generally never leave it in automatic shifting. I don't really use Sport+ or any of the other modes, just Sport and then the paddles. I find the automatic shifts to be imprecise and never what I'm looking for.