I'm not sure about the whole consistant RPMs thing, but I did have problems during a road trip this summer with the CTD 6 speed.
We drove coast-to-coast (west to east) in the 3500HD (no mods, it was loaded with a cover over the bed) for about 18 hours a day at a time (I'd drive about 12 hours, my mom about 6). After about 6,000 miles or 5 days of going a constant 60 to 80mph (2,200 - 2,500 RPM I think), we had a fuel injector give out on us on our way back west. During this time all of the gauges such as engine temp, oil pressure, etc., were reading normal values.
I think that my experiences were unusual and not reflective of what most CTD's would go through at such RPMs, but I wanted to point it out anyways for you to think about.
As for max speeds...I've only taken it to 90 mph on a pass. The only benchmark I have for power/speed would be a 6-7% grade that's 7 miles long that the truck (unloaded) can only just reach 70-75 mph WOT in 6th...and that's when it flattens out for a corner...