Want to know if you will lose tail authority by shortening the rear of the fuse? Use an online app like geistware.com has and you can play with all the numbers they ask for.

BTW, while shortening the tail sounds like an excellent move for your present design, consider lengthening the nose a bit as well.

Also study your empenage construction. and the aft fuselage design... Lots of ways to make things lighter. The current crop of ARFs have one good feature. They have sensible sized holes in lots of places. Holes don't weight a thing.