I don't think A2G attack pilots set their convergence for long ranges. In the book "Red Star Against the Swastika", the author (WW2 IL2 pilot) mentions that convergence was usually set at 500yds or closer.
Also, the 88mm doesn't adjust convergence on the fly, all you're doing is adjusting the range of firing and the timing of the 88mm round (timed fuse on 88mm rounds, not proximity like the 5" guns on ships). You could not adjust the convergence on the fly while in the air in a fighter in real life, it had to be done on the ground.