No. You don't switch for speed. You switch for comfort and to reduce strain.
The reason why the 120WPM speed is brought up at all is to (1) demonstrate that the keyboard design isn't totally bunk, and (2) that the person has spent enough time with the keyboard to get to a level of mastery with it that their opinion on the keyboard is coming from a place of knowledge.
What I have in mind is given some representative code sample, how many keystrokes are needed to perform a set of common tasks like duplicate line, reverse the order of function arguments, etc. There could be penalties for uncomfortable shortcuts like CTRL-B on QWERTY.
With cmd based keyboard shortcuts, your hand basically forms the default resting limp hand position, with your thumb on the cmd button, and your index finger lazily and easily finding the other key to complete the shortcut. Your thumb is basically always there anyway due to the spacebar. It's a much more natural movement.
If you need to keep your hands on the mouse (for example, CAD or video editing) then get a keyboard with programmable macro keys, or a separate macro pad.
For example, a Kinesis Freestyle Pro, or a Koolertron.
Whether the switching costs are worth it is an individual choice, and quite a few people have chosen that it's worth it. They're in the minority, but that's fine as long as people are free to use whatever keyboard layout they like.
If anyone is interested in switching, also check out the Workman layout, which is a slight improvement on Colemak's ergonomics at the cost of messing up some keyboard shortcuts
If your typing speed is high percentile, odds are you'd be faster than all your friends anyway, regardless of dvorak / qwerty. (E.g. I type faster than every single one of my qwerty friends! ...)
Given that <0.1% use dvorak, there's >90% chance you won't see a single dvorak user in a random sample of 100 people. Not hard to see why the leaderboards will be "dominated" by qwerty.
Like others have said, dvorak is for comfort, not for speed. Dvorak is MUCH more comfortable than qwerty.
Also, Dvorak users almost never complain that they type more slowly than their qwerty days, once they've gotten used to it.
Many of us dvorak users also are competent at qwerty in a pinch (80 wpm qwerty here).