Lance, I am in no way endorsing such an act. It has caused many to die.
As per stated above, combines are just left to their own regular speed. The rear-steering is unstable at higher speeds.
Matt, even if you did tweak the switch and add as much as you say, it's not really gaining much and for a combine, even just 4-8 more MPH can make a huge difference in stability.
Just drive safe. Take your time, guys. You're NOT going to make those machines move anywhere near as fast as normal traffic, anyway.