John in NM
I'll second Mark's suggestion of the 24 grit disk. I use one like this a lot and it really will hog off the wood. Snap a chalk line and cut down to it.

I wasn't entirely serious about the plaster, btw.... it does take a while to learn, and a lot of shoulder muscle. Adding vermiculite (if you can find it) helps a lot, both with the weight and somehow it makes it stickier. Although we grumble a lot about studs not being uniform - and always getting smaller - I think we likely have it a lot better than our predecessors with rough sawn stock.