ok, right. I got your meaning now. Correct, that is the point of it all... hold the gear as long as the thrust to the tire's contact patch is more than if you were to upshift... or run to redline.
...much as in the graph you see here for the "DRIVE POWER AND LOSS" graph:
http://www.club202.com/forums/showth...er=6#post72660
I started that thread as 98c43amg and changed my handle to c55m8o after the engine swap... I did this using CarTest2000 w/my C43 AMG after the 5.4L transplant from an E55 using measured torque on the chassis dyno for all the parameter sensitivity analysis.

 

Yep - looking at the thrust-versus-road speed chart for your Mercedes, it looks like you're in the same boat as the MINI:



Since none of the lines ever cross (except that the 4th gear line might *just* be touching the 5th gear line at about 150 MPH), you're better off just holding it in gear until redline, rather than shifting early. And the "thrust" on the vertical axis in the first graph is just wheel torque divided by wheel radius, which is why I like to think in terms of torque rather than horsepower right from the beginning.