You mean a 20" rifle?
It makes a world of difference in the ease of handling, about like the difference between a fixed stock 20" and fixed stock 16".
I like the LMT receiver extension, you can get it with buffer, spring, and castle nut from ADCO firearms.
As for the stock, I went with the standard LMT stock (the $55 one) because I was not sure I would want to keep it on the rifle. It works so well I am keeping it that way instead of going to a Magpul M-95B or VLTOR modstock.
You can put one on an AR-10, rifle or carbine, so long as you are using parts designed for the AR-10.