The D800 in DX crop mode does not change magnification or anything else other than a black outline of sorts in the frame.
So if you put a 200 lens on it that is DX, and look through it, and then do the same from the same place as your D300, you see the exact same screen (inside the boxes - there is some bleed over), but the angle over which your eye sees it is smaller by the same proportion.
I have no idea if a magnifier is available. It's not an issue for me, as I don't use the viewfinder for focus. But it does take getting used to, especially if you use DX crop mode with an FX lens where you don't get the darker field to remind you that you are shooting only in the center.
There's always live view if you are doing something that needs magnificantion (like focus). And it's a better live view than the D300 in several regards (easy one button activation, better focus, bigger screen, and no mirror slap).