I found a blog post that did a similar study to the one that you are suggesting:
MoneySavingExpert.com: Mastercard vs Visa for using abroad - which wins?
In it, there is a graph that compares Mastercard vs. Visa rates for an entire year spanning May 2014 - May 2015:
However, if you look at the graph, you'll see that they only actually have 2 points per month. I suspect that the blogger got her data from the same Mastercard and Visa pages that you found. You'll also note that Mastercard consistently beat Visa during that year.
An older blog post did a similar comparison in 2013:
theinquiringinvestor.com: How does Visa Calculate its Currency Exchange Rate?
There's no pretty graph, but that blog post not only compared Visa vs. Mastercard, but also compared other exchange rate sources. In 2013, Visa was consistently better than Mastercard, and Mastercard was consistently worse than all other exchange rates. The post got a 2015 update, and the author acknowledged that Mastercard now featured rates that were consistently better than Visa.
The blogger also mentions that this data came from those Visa and Mastercard pages you found.