I'm not sure.
Judd Trump is a great shooter, and did a credible job playing 9-ball, but wasn't anywhere near his capabilities in snooker. In the same way Corey Deuell or Alex Pagulayan didn't perform to scale when they tried Q-school in snooker.
Thinking about these three players where we have video, I think the performance is some combination of shooting, cue-ball control, strategy and tactics, and luck. All these players are great shooters and have great cue-ball control, so I've assumed the difference is knowledge of strategy and tactics. I don't have any sense for how long it would take Trump to get to a top-16 professional level in 9-ball, assuming he even wanted to. But i suspect it might take a year or two.
I don't know enough to have a good feel for this.
Are there excellent 9-ball players that aren't so good at 8-ball? Or vice versa?