No, they are all tested to the same specs. Here's what they do in theory:
1.) They test a CPU at a given speed and voltage, if it fails, they lower the clock speed until it quits failing, then lable it at that speed.
2.) Processors have to meet thermal guidelines also. Some processors will run at a given voltage but be hot. It almost seems certain that these will be assigned a lower clock speed as well.
I hear that CPU's are "batch tested" for speed. I'm not so sure about that. But if they are batch tested, it would be required to take a large sample and set the whole batch at the lowest speed applicable to that sample, with some margin of error.
The letters you see assigned to various processors refer to core revisions. During the production of a given processor, people will study things like "what part runs the hottest" or "what part is most likely to fail at a certain speed", then they will make very minor design changes to fix these problems, so that subsequent revisions run cooler and clock higher.
As far as I know the CPU testing is on a "Pass/Fail" system, where CPU's that fail even at the lowest speeds are scrapped.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>