Speaking as a player that has fluctuated between 2050 and 2150 for the past few years, I can tell you that I am pretty booked up.
In theoretical openings like the King's Indian Defense, Mar Del Plata variation, I could play, assuming White doesn't deviate and cause me to spend time to think about the issue with White's move given various King's Indian concepts, roughly 15 moves in under a minute. Now, of course, a GM that plays the main line Mar Del Plata will probably know it 30 moves deep instead of that of an expert who knows 15 moves.
I can play various lines of the French 10 to 15 moves deep without thought, especially the MacCutcheon.
Another difference between a 1700, 2100, and GM is a 1700 may be booked up for one opening. Say, the King's Gambit. If you play c5 or e6 on move 1, they are merely guessing what to do, and you can tell from their play. A 2100 player may be booked up in certain openings or certain variations of certain openings, like a know one player that can play 20+ moves of King's Indian Mar Del Plata with no sweat, but you play a Saemisch and he goes in the tank in terms of time. Then, at the GM level, they know almost every opening, and surely every main stream opening, pretty much inside and out.
So, for example, if you were to face me, you'd probably be in major trouble if it was a King's Indian or French, myself being either color. If you are White, your best bet against me is go for something like an English Opening. If you are Black, and you play a French, look out. But if you play 1...e5, you've got pretty good chances against me, and hence why I am an expert and not a GM. Mastering just the French and King's Indian is not enough to master the game of chess!
Reason for the Question: I "retired" from OTB about 30 years ago with a 1762 USCF OTB rating. I am planning to return to OTB play, possibly the latter half of this year. Obviously, I am way over-rated. I don't even know how I got to 1762 since I never studied openings. (Lines after lines of Figurine Algebraic Notation just make me want to throw up.)
Now that I've "re-discovered" chess at an age where Garry Kasparov and Yasser Seirawan have retired (my age co-horts), and yet I want to play! again, I'm wondering how Booked up today's players are.
For the USCF commenters who still play rated OTB events, how booked up are players in the 1400-1799 range? From 1800-2100 range? I know I'm gonna drop rating points knocking the rust off, but I don't want to hemorrhage them like a bleeding trauma victim either.
I know there's a range, but I'm wondering if there's a large majority of players who are booked up. I suppose I should define what this patzer means by "Booked Up." A Booked Up player is someone who has is repertoire down to where he knows the first 6-10 moves of his opening and knows the ideas and themes of his opening choices.
I am at a distinct disadvantage to these booked up players when all I try to do is to try and reach a playable middle game.