Now, anyone can zoom in with their browser... but your entire layout changes width when I do that: http://i.imgur.com/U96Py.png
This seems the most trivially easy thing to A/B test. Does anything less the 16pt cost you revenue?
Simple question, yes/no.
Actual data is so much better than someone's untested opinion which has been shown time and time again to be worthless in real life.
That's how it looks in my browser:
http://freeimagehosting.net/newuploads/cyqjo.png
(Firefox 18 / KDE 4.8).
Example (from this site):
font-family: 'proxima-nova', 'proxima-nova', 'Helvetica Neue', Helvetica, Arial, sans-serif;
proxima-nova and Helvetica Neue are not common fonts, so putting them as first with fallback to Helvetica is not the best option, if the font isn't provided as WOFF.