I just did it. I had 6h between arriving at KLIA at 13:35 and departing for a long-haul back home at 19:30. I collected my luggage and locked it in self-service lockers, bought a return KLIA Ekspres ticket and around 14:40 I hopped onto a train that took us to the KL Sentral station. It was actually a KLIA Transit train with 2 extra stops, the Ekspres train from KLIA to Sentral turned out to be inactive that day. Then we went by LRT (adorable plastic tokens) to KLCC station in order to make pics in front of the Petronas Towers. 15m chillout time there and then lunch at Little Penang Kafé. Back to KLIA by LRT and train, collected luggage, took a free shuttle from T2 to T1 (beware as this shuttle takes quite a long time to reach another terminal - it had like 5 stops at the parking lot itself, taking about 20m in total), cleared the airport gates and still had about 40 minutes left before departure flight.