Re: How good is someone according to MOA?...
The answer to your question depends a lot on the location (ie. the shooting conditions) of the match. For example, couple Sundays ago, I shot a 3 x 600 match at Camp Pendleton, which typically has the wind coming from NE in the early match, S or SW in the 2nd, and SW much stronger in the 3rd. We were fortunate to have only about 1 mph wind for match 1, but true to form the wind came up fishtailing at 10-12 mph from 6:00 to 8:30 for the second and slightly more consistent at 10-12 from SW in the third. My scores were 200-11X (my first ever "clean" by the way), 184-4X, and 195-10X. I'm sure you can guess from my scores where the fishtailing winds were predominant.
Same thing yesterday in a 3 x 1000 match yesterday at Camp Pendleton, although the wind directions were reversed because the 1000 yd range goes N to S, which is opposite of the 600 yd range. At the 1000 yd match, which was only my second competition at that distance, I shot 194-7X, 193-1X, and 182-2X. On my 3rd round, the wind shifted almost 180 degrees for my last 3 shots, which were 7s. Thus the 182-2X. I saw the change and held off more for each shot, but it wasn't nearly enough. In any event, both of those scores were good enough to place first in F-T/R, where there were a small number of shooters as they are local matches. I even managed to score higher than several F-Open guys, for the same reason. However, at the regional or national level, I would regularly have my ass handed to me. The main point is that it's all relative.
Bottom line is that you can look around to see what others are scoring at competitions in different locations, but unless you're familiar with that particular range and the normal conditions, the numbers may not mean very much. When you see top national/international shooters dropping 10-15 points in a match, you can be pretty sure the conditions were challenging.
So how does that relate to MOA? As mentioned above by XTR, 90% in the 10-ring would get you a 198. At 1000 yd that would be phenomenal shooting if there was significant wind. At 300-600 yd, that would be pretty mediocre for a top level shooter and would doubtfully put them in the money. In general, if you can't hold MOA (more probably 0.5 MOA) under good conditions out to 600 yd, placing at or near the top will be a rare event. Under more adverse conditions and/or at longer range, the same level of precision might easily win the match, although the shooter that can barely hold MOA out to 600 yd under good conditions probably would shoot more like 2 - 3 MOA under adverse conditions, obviously getting worse as the range increased. Overall, I view it kind of like the US Open golf tournament. Every year, lots of the pros complain that the rough is way too long and the course is very difficult. Well, the way I see it is that a) it's the same for everyone, and b) if you don't like shooting out of the rough, keep your ball in the fairway. By analogy, the best wind readers will generally have the best opportunity to place in the top echelons, regardless of the final scores or what MOA those scores translate to.