Don't do it. The Sigma stem is a poor idea, and since it allows for no steerer projection through the stem and has a low stack, if you use less than about 10mm of spacing under it there's no turning back short of buying a new fork. That's why they're basically giving one away free with every Alpha purchase.
Is that really an issue once I've got it to fit me?
It's OK as long as you don't change to a different set-up later. I only mention it because somebody mentioned on the TT Forum that one of my slam-that-stems had saved them from having to buy a new fork after ditching the Sigma.
Don't do it. The Sigma stem is a poor idea, and since it allows for no steerer projection through the stem and has a low stack, if you use less than about 10mm of spacing under it there's no turning back short of buying a new fork. That's why they're basically giving one away free with every Alpha purchase.