Unless a startup is doing something absolutely terrible, a 45% increase is just not likely. Your advice could better be rephrased as "if you are seeing low enough numbers that it effects the viability of your company, redesign X and then A/B test the change."

Startups have a hundred better things they could be spending time on. A/B testing preys on a founders sense that they are leaving money on the table and not optimizing for maximum profit, sometimes to the detriment of focusing on things like good hires or critical product changes.

Edit: It isn't obvious until I clicked around, but the author is a little biased and should disclose he is the "Head of Optimization" at Optimizely