Yes it's absolutely fine that China will slow down, as long as it doesn't completely stall out. It has a very long way to go to catch where Taiwan or Korea are now, though, let alone catch wherever they or the US will be in another 20 years.
Singapore is a city state and thus comparable to major US metropolitan areas, which generally have GDPs per capita in the 70s.
You've got wrong data, or something other than GDP per capita. The US was in the 47-50 range in 07-11, Singapore 39-53 (and behind by 5-year average), Korea 18-24, Taiwan 16-20.