I started working on this idea years ago before there were any other options. I tried 2", they really didn't help me so I went to 3", again some help but I still had pain in my neck & shoulders. I finally settled on 4" mainly because that was as big as I could swing in my lathe and I didn't want "ape-hangers". This version was comfortable for me. I'll include some photos. The last photo shows my riding position. I'm 5'-10" tall, 32" inseam, wear a 15" X 32-33" shirt.
They do spread the bars but that's not a bad thing, since I also made the wedges I combined them and the resulting tilt to the base of the stanchions puts the bars nearly the same spread as stock.