Kenmac -
I think the Stihl BR600 and the Echo PB-751 are in about a dead tie for most power, from what I've seen. That was 3 months ago, something else might have come out in the meantime (like the Shindaiwa). As you noted, there are a few different ways of rating blowers' capabilities: MPH of the airflow; CFM at the housing; and CFM at the outlet. Stihl's numbers might be slightly higher than the Echo, but I've tried them both and if anything the Echo seemed to do a little more actual work.
My bottom line: They are so close in power that the edge goes to design and ease of use. I think the Echo is the nicer of the two machines. And this is coming from a Stihl man. I LOVE Stihl - just not for everything. I have, and have had, a STIHL 066, 046, 036, 031, 028, 026, 011, 009, MS200T and MS191T. Different manufacturers just seem to hit the nail on the head in different parts of their product line.
What I like about the Echo: Easy, easy starting, easy to work on, GREAT air filtration system, super-long lasting engine (part of the air flow is directed over the engine for cooling), trouble free and easy to use. Again, not knocking Stihl at all, or Shindaiwa for that matter.
There might be better/more powerful blowers out there, but I urge you to try out an Echo 751 before you buy.
Good luck, John