I went down this road several years ago with a 3.9oz aluminum carrier, empty buffers, and adjustable gas. What I found was that the juice just wasn't worth the squeeze. Yes, there is a difference in sight movement when you go really far down this rabbit hole. Compared to an off the shelf, overgassed, carbine length AR the difference is tremendous. Compared to a properly built and gassed gun, not so much. There's still a difference for sure. The question I finally had to answer is whether I was able to exploit the difference. And for me, the answer was no. Sight movement between shots, comparing something like my 13.7 and that aluminum carrier build, was definitely not what was holding me back in a match.
And that little bit of advantage does come at a cost. Once you start fiddling and straying that far from the standard your window of "this works" becomes increasingly narrow. You'll have to pay attention to everything and keep an eye on ammo, weather, cleanliness, and lubrication more than ever before. All of that is a hassle and distraction that I wasn't willing to deal with.
As far as what parts you need to get, it depends on what your goal is.
If you want to minimize sight movement in recoil get a reduced power buffer spring, a carbine buffer, and adjustable gas block. Try it with the carbine buffer. If you want to go lighter I'd recommend replacing the buffer weights with appropriately sized wooden dowels, one at a time, and test it as you go. IME having the dowels in the buffer helped in reliability. Somebody told me it has to do with the way the weight reciprocates in the buffer vs having an empty buffer. I don't know, but it seemed to help. Remember that it's a system. You can't just remove all the weight without consideration to the gas. And removing too much weight reduces forward momentum of the carrier, decreasing reliability when you're feeding ammo from the magazine.
If you just want to build these guns and use the RTB carriers because that's all you could find... Get an adjustable gas block. Don't be lazy, dimple the danged barrel and install it correctly. Then I'd start with a carbine or maybe Springco white-hot spring and an H buffer. If I had one handy I'd try an H2 buffer as well but it's probably not necessary. Since the carrier is so light you'd want to go heavier to compensate for the missing mass. Then turn down your gas until your ejection is at 3 o'clock and your bolt locks back on an empty magazine every time.
You're getting advice to go both ways, and they're both right, depending on what you're wanting to accomplish.
