I've a 2001 500 Bullet, and to be honest, its happy cruising at only 55mph. Cruise at 60 for sustained periods of time, and I burn the exhaust valve, from personal experience. Now, just to confuse the issue, mine is the classic engine. It has since been surpassed by the leanburn engine which may have a higher cruising speed, and then the EFI engine which will definitely sit at motorway speeds. So, you need to know which 500 as well

The 350 will sit at the same speed as mine, and is, I'm told, a smoother engine.