But mentionning that wouldn't fit in the marketing speech of Google and Microsoft (which pulled _exactly_ the same stunt a couple of month ago, IE slightly on top of course)
Microsoft has actually released numerous studies through NSS Labs. However, I believe what you are explicitly referring to is the yourbrowsermatters.org campaign. That was not a study, it was a marketing site that mapped the browser's user-agent to a listing based mostly on past NSS Labs studies.
As for the study from Accuvant, I definitely encourage you to dig into as deeply as you feel appropriate. Unlike the pro Microsoft reports from NSS Labs, this Accuvant study is completely transparent and verifiable. The methodology is documented and all tools and data sets are provided. If you have a legitimate concern with its merit, then you should have all the information you need to address it.
Show how much truth there is in these "studies".