I can't confirm from experience, but there's no reason to doubt that the DIR-878 A1 will function at AC2600 speeds. The internal images and test reports at the FCC suggest AC2600 and the confirmation from D-Link is just the icing on the cake. I'd love to hear what the link rates are from someone who has one in their hands too though.
On a related note, the
D-Link DIR-867 rev A1 just hit the FCC today. It's identical to the DIR-878 A1 and the DIR-882 A1 but it's actually missing some pieces which cripple it to 3SS. This supports the idea that the DIR-878 was something that they could just relabel and get to the market quickly after the DIR-882 to capture a different segment of the market. It took some time to actually alter the design for this new DIR-867. Even if they could flip a software switch to restrict the speeds, that would require additional testing and delay their time to market even further. Imagine the efficiency and cost reduction of producing one board for three designs!
I wouldn't be surprised if the DIR-878 A1 is quickly replaced by a new hardware revision that is similarly (actually) crippled but still has 256-QAM to give it that beautiful AC1900 label.
