First off that is why I hate nearly all those stupid television shows. Guys that don't know d**k always refer to the guys on those shows as the experts.
As for your question, there is no one answer. Best answer would be "That depends". What kind of standards do you hold yourself to? What kind of standards does the customer have? It's all relative and dependent upon peoples individual perception.
Years ago I was part of a '56 Mercedes 300 SL Gullwing restoration that had over 6000 man hours on it and it was close to perfect. Five of us worked on that car for close to a year. Nothing else. But that was a total restoration done to a very exacting standard. I have a 72 Chevy Pickup in my garage now that was just supposed to be a fairly quick repaint that has now morphed into something that I'll have easily 600 hours or more in. It'll be nice but not perfect. But someone may look at it and think it's close to perfect. Just depends on what someone thinks is perfection. I have only seen a few "Perfect" cars. And I'd be willing to bet if I laid eyes on all of them now I wouldn't think they were perfect anymore.
300 hours isn't going to get you "Perfection". IMO that is more like what a really nice driver should take for final prep and paint. But like I said perfection is in the eyes and mind of the person doing the work and the car owner.
That being said if it was my job, 300 hours is probably enough to make it fairly nice. Or it might not depending on what you are workig on and the size of it. If I was just doing a really nice driver I would have 60-80 hours in cut and buffing. You could easily spend 40 hours getting everything gapped. (metal guys should have set the gaps BTW.) Blocking anywhere from 40 on the low side to 80+ hours. Paint depending on how many pieces you do it in, 40-80 hours. Re-assembly another 40 or so. Then I'd add 25% to account for my underestimating that gets you to well over 300.
All of this is just speculation though. I would count on things taking longer rather than shorter. And in 2019 $20k for a paint job really isn't much more than average.
OH and I hope this won't be true but judging from what your customer said, more than likely he is going to be trouble.