From the perspective of somebody that uses the 51 a lot, there are less than a handful that are extremely good right now. There are a lot of very good 51 guys, but most will fly 'smart' and leave the engagement when they lose their advantage. The ones I consider extremely good are those that apply great cartoon-ish ACMs to their 51 while stall-fighting a superior turning enemy.
Most people can learn to be very good in the 51, come in with some type of advantage, and win the fight. Much fewer develop the skills to turn with 38's, Spits, 109 variants, and so forth.
From my flying in the 38, only one person in recent memory has stall-fought me and won while he was in a 51 and me in a 38. It felt like I was fighting myself. I also haven't fought 99% of those on Knights, as I've only changed countries a handful of times over the last year or two.