My figures come from over 100 transactions across 8 currencies including some of the most commonly used non-US currencies, GBP, EUR and AUD. Others in that thread have reported similar results. If you know exceptions, by all means point them out.

It does not "even out in the wash". Better to look up Visa's rates on their web site and compare to interbank rates on xe.com or oanda.com yourself rather than making such a ludicrous claim. On my data, Visa is worse and very consistently so. Worse mean. Worse maximum. Worse minimum. In transactions with identical date and currency, pr(Visa betters MC by >0.2%) = 0 while pr(MC betters Visa by >0.2%) is over 90%.