With my 2011 using Manual defeats the "grade control" of the Allison transmission from working properly. With Tow Haul mode engaged when I hit the brakes the engine shifts into a mode where the transmission downshifts and there is no fuel fed to the engine to achieve maximum braking. I get an additional 25-30% engine braking by enabling the exhaust brake.
The Duramax has a maximum RPM that the computer will not allow to be exceeded so no worries if you see the engine at over 3000 RPM on a steep downgrade. It is operating as it was designed to do. The LMM engine produces maximum horsepower at 3200 RPM which is within its safe zone.
I would add a third-party brake controller regardless of the affect on the drivetrain warranty - what is your life worth? At least as of 2011 a brake controller was included as part of the tow package with the Duramax trucks and the exhaust brake is provided with all Duramax engines.
