Borys was a knight and participated in tournament melees, and he had a known history of fighting in war, I don’t see why he wouldn’t have been a great warrior lol.
And again this is more about Jahaerys himself. You also had Ellias stating that Jahaerys would defeat Maegor. Jahaerys was taught how to fight by seven superb Kingsguard knights, and Jahaerys won both of the noted 1v1s that he fought in, one of them was when he was 55 fighting a 17 year old. Add into that that Jahaerys killed a veteran Baratheon knight 1v1 and this all paints the picture of Jahaerys being an excellent fighter.
Again this is all based under the impression of he was a monsterous fighter.
Which he was not, ser arlan also fought in tournaments and wars. I wouldn't call him a legendary knight at all.
Jaeharys was never regarded as a great warrior.
On top of that, when his master in arms said he could beat maegor now. Jaeharys was just 15 at that time.
Meanwhile Maegor is who you call a legendary fighter of his time, fought and won many duels.
Personally trained by or under visenya. Holder of dark sister which are only given to exception warriors like Visenya, Maegor, Daemon, Aemon, Blood raven.
He had two duels in his life, no tournament or anything. One was against a beesbury and other was against Borys baratheon which probablt wasn't even a fight, Jaeharys ended that very quickly in a single blow.
So either Borys was a low grade fighter or jaeharys was an exceptional fighter like hound or mountain or something given the feat. But that's not the case at all.
Overall he was an above average Fighter, really put his name beside the kind of Arthur dayne, Barristan, Jamie, loras, obrien, aemon, daemon blackfyre, daemon targaryen, criston cole, Maegor targareyan etc.
He was never of this caliber kind of fighter.