I can’t say for sure, but from what I could gather from my hours of play time, training moves to the max is a complete waste of time and gives you nothing in return (other than that slight XP boost when transferring the save to Shenmue 2).
You already seem to be familiar with the benefits of training moves to intermediate level. Each move behaves differently in this regard, but in general, moves can:
[1] Get a damage boost;
[2] Evolve into a more refined form (with different animation, collision and range. This is not always a good thing, but it’s irrelevant to the topic);
[3] Get a unique follow-up;
[4] Get a combination of 1, 2 and 3.
To make some examples, Brutal Tiger, Rising Flash, Mud Spider and Thunder Kick all get [1] and [2]; Mistral Flash, Windmill, Twin Hand Waves and Elbow Assault get nothing at all (or maybe they do get a damage boost, but it’s insignificant); Crawl Cyclone and Stab Armor only get [2] (yeah I know, Stab Armor’s intermediate version seems stronger, but nope, damage is unchanged
).
Maxing out the XP bar on the other hand is a different story entirely, because there are no changes to be seen at all. If Shenmue 1 featured health bars for opponents, it'd been easy to spot any difference in damage output, but since that's not the case, I was led to believe nothing actually changes after reaching intermediate level/50% XP.
A couple of years ago, I did some tests in the 70 Man Battle with two different save files: Save 1 had Ryo’s initial moveset with 0 XP, while Save 2 had Ryo’s end game moveset with most moves between 55 and 70% proficiency. When fighting Shingo Murasaki for example (the 1st boss you face), it’d take 4 Demon Drop/Darkside Hazuki to knock him out with Save 1, but only 3 Demon Drops/Darkside Hazuki with Save 2. When fighting the weakest Mad Angels you can encounter, 2 Tiger Knuckles were enough to defeat them in Save 2, but not in Save 1, where one extra Tiger Knuckle was needed. In short, it was clear moves were stronger in Save 2 due to me playing through the game and spending time in the practice training mode.
I then tested Save 2 with a different save (we'll call it Save 3), where I had some of Ryo's moves at 100% proficiency. I could not spot any differences whatsover, until... I used Dark Moon.
In Save 2, Dark Moon was at like 65% XP, and when I went and used it against the toughest non boss opponent, he would barely survive it, and any following attack would defeat him. In Save 3 however, where Dark Moon was maxed out, that same guy would go down with a single Dark Moon, no extra Tiger Knuckle needed! I was shocked!
This kinda hints at the possibility moves do keep getting stronger after reaching intermediate level, but even if that’s the case, the damage increase is so insignificant it’s almost impossible to notice.
For example, I once maxed out all throw moves on the PC version of the game by using a trainer, and when I went and tested their damage outputs and compared them to the legit save I was using, I could not spot any difference.
In conclusion, there's evidence that suggests moves might get a very slight damage boost if they reach/get close to reach "master level", but I don't think it's worth going through hours of tedious grinding just to obtain a supposed damage boost you might not even notice during play. I had to spend hours setting up saves with very specific training levels and test moves against specific opponents just to notice Dark Moon gets an additional damage boost between 50% and 100%!