Thanks. Wouldn't this need to be done simultaneously? For example, serving half your users methodA and the other half with methodB.
We don't know if their SD is in the same ballpark. Thousands of possible confounds. Plus, there's no solid a priori reason why shaving off latency should improve their conversion rates drastically -- Basecamp doesn't rest on a large number of small, potentially impulsive transactions like Amazon does. Without more data (or at least an explanation), this doesn't tell us anything.
My main point was simply that I don't think it's prudent to create a worse experience for half of your users when you're so unlikely to gain any actionable information from it. It would be quite extraordinary if they found out that the speed increase caused a decline in conversion or a huge improvement in conversion, so I think it's safe to say that the measurement wouldn't make them reverse the change or make them allocate significantly more resources toward faster load times.
But, to reiterate, I agree that they should not have made the claim about 5% conversion when it wasn't properly supported.
The hole here is whether or not they unknowingly got a new influx of traffic that was 5% more likely to convert, skewing his final observation, which I would say is unlikely. Your point is good in general, however.