I think milling the rear sight base off would be a mistake. The miniscule amount of weight dropped would not be worth making that permanent change to the rifle.
I think lightening the stock would get you more weight savings. Is the new buttstock bored for the cleaning kit? If not you could drill lightening holes and slim dwon the butt quite a bit.
If the clips don't work to your satisfaction, remove the stripper guide.
For what it's worth, I've fired my Scout without the muzzle break attached, and for me anyway it is worth it's weight. It is very effective in reducing recoil and percussion.
For what it is, the M1A Scout is pretty minimalist right out of the box.
All that being said, it is your tool and you must make it do the job you need it to do for you.
Interested in following you project along. Keep us posted on how it goes.
