I read an article a year or more ago that stated the max rpm for our engines is 6400, but I cant find the article to confirm it. Members here seem to set the rpm limit at 6200-6300 rpm.
I just found this interesting comparo of a c32 to a c55 with engine specs......c32 max rpm was 6,220 for 1 minute. Interesting.

This article also mentioned that the c55 has a "manual" mode added to the transmission electronics that allows it to hold whatever gear you select. This would be a nice add on feature to the Xfire!

Mercedes-Benz Specification Site C32 / C55 comparison - Marcus Fitzhugh

Cheers,
KrzyFast