OMG I was just going to post the exact same problem, I did the same and mine shows up as 400 MHz too, but like you I can feel the difference but I want it to say 600MHz in profiler just so I can actually celebrate a job well done.

I also edited the Open firmware to no evail,,,I wonder if there's any other way to adjust the speed,,,,

let me know if you figure anything out,,,I'm starting to doubt myself and wonder if I did it right.