I should also note that we constantly test and rotate about 10 different landing pages, optimizing towards the ones that convert best.

That's interesting. However, in the design community fab.com seems to be spreading by word of mouth, so in my own case, I signed up eagerly on a recommendation and I don't recall spending any time reading what the landing page said. Is there any way to account for that sort of signup?

It really depends on how you measure conversions. If you measure actual sales, that's the way to go.

If you just measure signups, I can come up with a page that will convert 100X better in a second. "Get $1000 when you sign up".