cylinder size makes no diffrence on the type of gas used. your bike will still run just fine on regular as long as you dont have a high compression kit or a shaved head on the motor. read this link, it will help you understand a little better. HowStuffWorks "What does octane mean?"
as far as how fast goes, you wil feel a power diffrence but it is mainly the returning power that has degraded over time. the more worn your rings and cylinder walls get the more compression you lose. a fresh rebuild will feel like a new bike. but dont expect it to be much faster than a new bike. if any at all.