Petrosianic: <20. Nf5 followed by f4 and g4. I prefer white.>
In a club game, it might be worth continuing. Among GM's, after 20. Nf5 Bf8, and Black is rock solid. With Queens off, a kingside attack isn't going anywhere.
Black might even be a little better. 20. Nf5 Bf8 If 21. f4, as you suggest, then g6 kicks the Knight right back again. To where? h4? g3? Neither looks great. Black reponds with Bd6, and the f pawn is a target. g3 and then the h pawn is a target. g3 and h4, and White's kingside is more porous than it needed to be.
Or maybe 21. g4 first, preventing an immediate g6. Black plays 21...Ng5 and White's kingside is porous again. He probably has to play 22. Bxg5, giving up the two Bishops, and forget about any White advantage. Or 22. Rh
1, but it's hart to advance in a meaningful way.
I prefer Black, if White tries your f4, g4 plan. But if he doesn't, it's about dead equal.