Whether to go into town in Singapore all depends on the queues at immigration. If you arrive alongside a couple of A380s then you can be waiting an hour to enter the country and that would make your margin too small but sometimes I walk straight through... so I would suggest not setting your heart on it, head straight to immigration (with completed landing card) and if you go straight through then a taxi to down town will take 30 mins max, you can chill for a couple of hours and still easily make it back to the airport with time to spare, security is done at the gate and immigration on the way out is now very quick because of e-gates.