US high rise handle bars are just Hippy type bullshit. When you sit vertical while riding a motorcycle the bumps from the road go straight up your spine. When you lean forward, your spine is more flexible and less likely to be damaged. American motorcycles are not designed for fast riding around twisty bumpy roads. For speed, you are always better to be down a bit out of the wind.
One of my mates had a Norton Manxman, which was effectively a 650SS - it had high bars - American market. Otherwise it was brilliant.
Easy Rider and The Wild One were American, Brando's bike had low bars - the high ones came later.
When Poms ride motorcycles, they usually mean business. When Kenny Roberts raced in the UK, he was accustomed to getting on the loose stuff, so he did OK. when I was a kid, I could have gone there. The onboard videos of the IOM horrify me. Australians such as Doohan etc. have beaten the Poms. But Ken Blake was one of our best riders - I think he only managed to complete two laps of the IOM. - Riding too fast !