That said, why not give it a shot for a day or two? At worst, you'll realize it is going to take forever to regain speed - at best, you're well on your way to using the new layout.
They don't even try do to exploratory programming. In cases where I would just write some throwaway code for an experiment, or turn to a REPL session to answer a question in a few seconds, they start leafing through documentation or Googling for someone else who knows the answer. I write and re-write code while I'm thinking; once they start writing they are unwilling to change their minds mid-stream. Even in normal "batch" programming, it means that instead of 5 minutes of thought followed by 1 minute of typing, they have 5 minutes of thought followed by 3 minutes of typing - so they take longer to resume their stream of thought, and spend slightly less time thinking overall.
*I type about 60wpm which is not especially fast, it's just that I work with some people who are not touch-typists.
I'm not sure that a valid comparison / assessment can be made after a day or two's use.
Typing is definitely the bottleneck for me in some situations, especially when I'm prototyping something, and I can hit 100 wpm without too much difficulty.
As for giving it a several day test drive, I don't think that's going to help at all. The point is that once you know dvorak it is faster than qwerty. It would probably take a least of month of dvorak to be able to make a reasonable evaluation.