Heavy buffers will lengthen the recoil impulse some, but they will increase the total movement, albeit very minimally. Yes, a better muzzle device is a better choice.

The majority of 3Gunners use reduced weight buffers and reduced weight carriers with good comps to keep the rifle flat and fast. The next step is to look at gas volume and pressure. An adjustable gasblock to reduce the excessive amount of gas delivered will help. A rifle length gas system with a smaller than mil-spec hole and a solid gas block is a better more reliable solution.