The speed limiter is SCN coded in the ECU.

For the NA ECU, the available values are: 160/170/180/190/210/240/243/250 km/h
and for the SRT ECU, they are: 160/180/210/230/240/250/280/300 km/h

In all the Crossfire ECU's I've looked at, the speed limiter has been set at 250 km/h (155 mph), for both the NA and SRT6.
So, if you wanted to, you could increase the SRT6 limiter to 300 km/h (186 mph)

You can change them using Developer mode in Mercedes Star/Xentry.

*** Just as an addition. I was reading some old posts in which people had adjusted their measured speed because they were running bigger rear tyres than stock. The speedo reads from the front wheels.