The short fat 284 case is more efficient than the '06 case and handles pressure better, therefore you get higher velocities out of it. I have built many, many 6mm-284s and it is a super cartridge, anyone who says it has nothing on the 243 has never had one, obviously. With most bullets is has 400 fps on the 243 and I've yet to build one that didn't shoot sub MOA with several bullets and loads. I have never had a 6mm-06 or known anyone who did, but I can tell you that every wildcat I've seen does better on the 284 case than on an '06 case, 25, 6.5, 338, 35 and 375.
As far as brass goes the best is out there, with Win still available in original 284, then there's Lapua in 6.5-284 and Norma in 6.5-284 and whatever brand Nosler is marketing. There is no shortage of good brass, albeit none of it will be free like thousands of '06 are, if you don't mind mixed headstamps.
I've converted many 243s to 6-284 and never had to change a thing or work feed rails, they have all fed 100% just as they were and the same thing can be said of the rifles I rebarreled, never had to do any action work at all. I have fired thousands, if not 10s of thousands of rounds from 6-284s and never have I had a feeding problem. All of my own have been short Remington actions, but I have done a couple on 98s and a BSA as well as an old push feed mod 70 and a couple more I can't remember, as well as probably a dozen SA Rems.
I've even taken 4 caribou with my 6-284, one shot each with a 95 gn Nosler BT. They work fantastic in the wind for varmints, way better than slower 6mms or 22 cals............I can't say enough good about this wildcat cartridge, I have one in my vault now and a second long barreled 243 that may get the treatment one of these days. After all, who needs four 243s anyway..........did I mention I love 243s?