To summarize:

1. No visa for US citizen
2. Immigration is relatively fast
3. KLIA express is fast, 28 minutes but factor in wait time for next train, 15 minutes or so.
4. From Sentral, train or taxi to Twin Towers will work - another 15-20 minutes depending on wait for train, traffic, etc.
5. 3 1/2 hours is cutting it close. If you figure one hour each way, 30-40 minute sightseeing, that gives you 30-40 minutes buffer time to get through immigration on the way out plus return check in, go through security and immigration before boarding....but doable.
6. IMHO, airport is far from a Changi Airport but it is modern, sleek and stylish. If you're on MAS, they have excellent lounge, the others are comfortable but nothing to brag about.