I really don't think there are enough disc golfer out there to really answer this question properly. The more "natural" athletes start to get into the sport (which is already happening more and more) the harder it will be IMO to be a 900, 950, 1000 and 1040 rated player.
To take a stab at it......because I did play a lot of ball golf when I was younger, I would say the largest separation is putting and the mental aspect. The guys that are at the elite level are just downright fearless when it comes to putting......I'm a 943 rated player and I see those 1000 plus guys run putts that I wouldn't dare try to actually make (I'm more happy laying up for a tap in).....just my two cents.
"Apparently you need video evidence when you design a disc golf course these days"