Hello Rob, thanks a lot for showing us these test results!
Maybe you could make them public for all on the Serial reference page, I am sure others are interested in these numbers as well!
For now I ordered a Teensy 2.0 since I will need a central place from which to share the data of several Arduinos to the computer anyway, and I will get back to this problem and see how the Teensy Serial performs once it arrives.
Thanks again CrossRoads, Graynomad and Rob for all the advice, it is much appreciated!
Have a great day!
Tom