I think the sweet spot for Singapore's population is 4.9 million.
Surely a reduction of 400K people from the current population of 5.3 million won't impact the economy that much?
Less overcrowding. Less stress on the infrastructure. A moderating effect on housing prices. A more conducive environment for aspiring baby makers.
Remember, there is no such thing as a zero sum gain. What you gain in GDP from boosting the population number, you will lose in other ways.