I agree with you regarding statistical confidence. In this case, there was a second metric I was also looking at (which I didn't mention in the post and should

I agree with you regarding statistical confidence. In this case, there was a second metric I was also looking at (which I didn't mention in the post and should add) - the conversion of people who hit the signup page to signup complete. With this call to action it was improved 30% (+/- 5%) so I was pretty confident it was a win all around.