I've read on the forum from one of the testers that they have a 20Mb down/10Mb up link and everything works fine.
I think the game works on a tile-based pre-load (it downloads the terrain around you in advance) so theoretically, at least in small and slow aircraft the bandwidth requirements should be really low.
I would be surprised if the continuous downstream was anything more than a couple hundred kb/s. Kind of like Elite in Solo mode. They can't afford to make it too demanding.
Now multiplayer could be a whole another can of worms..
So the SR-71 might require Fiber
