I'll be in SIN and was thinking about a day trip to KUL, but not having been there before I don't know if it's a fools errand with immigration inbound and outbound. I see there's rapid transport to the city center so if I land at 9 am and depart at 9 pm, how much time will I have realistically have in KUL city? It also appears the "must see" areas are very spread out so I may not get much accomplished.

I won't have access to any priority lines and will be traveling on a US passport.