Valve says full Nvidia GPU support for SteamOS is a top priority and making steady progress, with drivers now in the main tree enabling some cards (though performance still lags).

also, like, if they are using nvidia proprietary drivers there's nothing they can really do to improve performance. AMD linux driver works so well because it's open source and some of the main contributors work for Valve. if they aren't going to use open source nvidia drivers they'd have to kneel before Jensen Huang and beg for a trickle of performance gain 'cause they (steamos) are like a tiny bit of nvidia's market.