UPDATE: We spoke briefly with a Microsoft Spokesperson. The official comment is: “On Xbox One games are games. All games follow the same policies for achievements and Gamerscore.”
Evidence has recently surfaced to lend support that all Xbox One games, both retail and digital, will launch with 1000 gamerscore.
First, Popcap Games tweeted out in a reply confirming Peggle 2 will have 1000 gamerscore at launch. Peggle 2 has recently been delayed to December, with no word yet on when exactly it will come to Xbox One.
Second, 343 Industries released both the Xbox 360 and Xbox One achievement lists for the soon-to-be released Halo: Spartan Assault. With the 360 achievement list, the game will still be restrained by the current XBLA achievement cap of 400 gamerscore. Contrary, the Xbox One achievement list will will have 1000 gamerscore upon release, with an additional two achievements exclusive to the Xbox One version.
Both these digital-only games will release with 1000 gamerscore like all retail games today. This new cap is probably due to the elimination of the XBLA brand, with all games on the Xbox One being grouped together simply under the title of "Games."
It is important to note that both these titles do not prove all games will launch with 1000 gamerscore, Microsoft has not given any comment in this regard. Yet, this information is still important to note for all you achievement hunters out there.
We should have confirmation soon enough, as Xbox One launches in just over a week on November 22. We have reached out to Microsoft seeking confirmation and will update the post should we receive a response.