I’m guessing the OW2 engine update will be treated like a full product release, in which it will go through a full battery of min-spec testing to make sure it runs properly on the intended platforms at the time.
Historically Blizzard’s patch process (not just in OW) has incrementally degraded performance in their games, and of course Windows updates are a moving target that has sometimes conflicted with Overwatch’s engine, so it won’t necessarily stay shiny and clean forever. The initial release should feel like a fresh start in that sense, though, since it will most likely be replacing the current Overwatch installation altogether.