I disagree. I'm sure Schumacher can regular drive street cars fine, even though they are vastly different than his Formula 1 racer that he spends hours training in.
Case in point: I have a Kinesis Advantage and even typing Dvorak on it, AND I have a Poker, an IBM mini, laptops, etc with the traditional layout (and I type them in regular QWERTY).
I am fully efficient in both. I just happen to do 100WPM on one and 75WPM on the others because of the keyboards' efficiency (and my own personal limits). I never did more than 80WPM before getting the Kinesis.
Other users, including iMav have an Advantage and regular keyboards, and can switch back and forth. While learning a new layout, it's better to stick with it for a week or two, but once you learn it well, you can start going back and forth and soon you'll be a master of both, and you'll be able to objectively compare them.