It's sort of like the much-talked-about switch to reverse scrolling in OS X. You may think it's a big deal, but your brain makes the adjustment, and it just doesn't matter. Your vi editing efficiency should be pretty nearly unaffected. It might be very fractionally slower, since the navigation keys aren't on the home row, but .. I never notice it at all. It's definitely not far less efficient.
Not arguing that you should switch, incidentally. I could also touch type 100+ wpm on QWERTY. I never regained my original speed after switching. That's very common for very fast typists. If you don't put systematic effort into speed training, you'll lose some speed for switching. Not that it's that big of a deal.