To the question of what kind of a raise did you get when you received your PE license, for me the answer is zero. All I have ever received, in terms of extra money for the PE, is that my company will cover the cost of the yearly license fee.
I'm in an industry (oil and gas) that supposedly pushes as many of its engieers as possible to get the PE license and I've not heard any of my colleagues mention raises just for getting the license.
To the question of getting more than the standard 3% raise, I agree that the best way is to leave your job for a new company. I've done that three times and each time the raise was good, at least 8% and a couple times over 10%.
With one of my previous companies I was averaging about 6% for my annual raise. Of course, I now work for a company that hasn't seen fit to provide any kind of raise for 3 years.