I mow about 6 acres that has no livestock and is irrigated (I play the "green grass look nice" vs. "then I'll have to mow" game a lot). But in the spring I have no choice and have 3 - 6 foot weeds out there.
I always mow in L, for a number of reasons:
1.) I have lots of steep hills. Hit a bump going faster on flat, and you bounce. Hit a high-side bump on a hill and the faster you're going the more likely it will be that you'll tip.
2.) Long grass bogs down the brush hog. I also find that even when I'm in L that certain types of grass will get "swept", rather than mowed, if I go too fast. Patience helps make for a better looking cut.
3.) OK, I'll admit it. I don't have the horsepower a lot of the time and I get the same symptoms described earlier. The revs will start to drop. Letting them go down a 100 or 200 RPM doesn't seem to hurt, but once they hit a certain point the motor starts to really bog down.
But if you're on flat, have the power, and are happy with the way it turns out, put the pedal down and keep it there. It shouldn't do more than bang things around a bit more.
...Chris