Well if you look at the size of the Zeus II you'll find that it is about as wideas a Freelancer MAX and as long as a Valkyrie, though shorter than the Spirit.
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I think that gives us a pretty good idea about how fast and nimble it's going to be.
A lot more mass than the Vanguard or the Cutty Blue to move around so "nimble" will be only in relation to ships its own size.
I reckon it's going to fly pretty well in athmosphere though.
But even if the MR gets supercharged engines like the Blue the only result will be a higher SCM speed.
That being said the Zeus MK II MR can stopp all of the ships up there dead in their tracks if it gets the drop on them.
As for being stealthy about it... once it's in the game we'll see.
Stealth gameplay, not just the ship. ;-P