Doors and Windows - Double-hung window sill higher than studs

I'm remodeling my bathroom and have removed all of the old plaster walls. I'm not replacing the window though and that's where I'm running into the problem. The sill of the window over the weights is 1/4" higher than the surrounding studs. The previous wall nailed backer up to the sill and then the plaster was pushed across both sill and backer to produce a smooth wall. I need a way to even up the wall and the sill. So far furing has been the only answer I've found. So my two questions are: 1) Is there a better/different way to even these out? 2) What's the best material to put behind the drywall as the furing? I need something that isn't going to split out when I screw the drywall into it.

You could cut the sill back with a reciprocating saw, or you could add lathe on top of every stud. If you use lathe, your drywall screws should probably be 1 5/8 so as to go through the lathe and into the studs. If you don't use lathe, 1 1/4" screws should suffice. You will want to ensure that your framing is straight (use a straightedge) because sometimes in houses with lathe and plaster the studs were of varying thicknesses, which creates a problem when you remove the lathe and think you are ready for sheetrock and you find that the studs go in and out all along your wall.

I would guess that if the original tub is not being moved and it will be reused, you will want to ensure that you aren't changing the location the tile (or what have you) will land on the tub.