does this make any difference whatsoever to the output. I am guessing not here

Think about it, if you change the brightness or contrast you're changing the RGB values, and the normals are derived from the RGB values...

guess I should have thought about that more

You can do it, works OK but not great. Just make sure to renormalize afterwards or your specular may get screwed.

Correct me if I'm wrong, but doesn't adjusting the contrast of the blue channel make the bump depth deeper?

Yep, that's correct. Gotta be careful though, if you get too dark on the blue channel it can look really odd on the model.

I have three different Actions for this.

One auto levels each channel independantly, then backs off to 10% using the edit fade command. One does the same thing but only backs off to 20%, and the third backs it off to 30%.

It does indeed alter the normals when you change the RGB, but sometimes you want that, and it makes it read better in game. This works much better on organic stuff, I always leave my mechanical stuff as is, other than fixing errors.