The cynic/economist in me says that this would be a classic case of "waterbed" economics. That is to say, any pressure pushing down in one part of the system results in a subsequent rise somewhere else. If you define corruption as a particular localised phenomenon, then pushing down in one area and measuring that area looks like things have gone down.
But there are many undesirable things in our societies (network effects, self censorship, rich ruling families, artificial barriers and rent seeking,etc). I know I've had many conversations with my wife where I've said "if this happened in any other society/market, we'd call it corruption, but here it's just "the way things are/our governance system".
So to bring it back to Singapore and economic theory, I would posit that the appointment to those highly paid positions would become the point of corruption as the wages/power/connections increase, though we'd probably start to call it something else (having the right connections, belonging to the right families, going to the right schools).
I know, incidentally, very little about Singapore...I'm just making general theoretical predictions here...
https://en.wikipedia.org/wiki/Temasek_Holdings https://en.wikipedia.org/wiki/Ho_Ching
The former Prime Minister (Lee Kuan Yew) was in power for 31 years. His son is in power now. People don't hold on to power for that long by playing fair.
Singapore is also known as the Switzerland of the East, not least for the fact that their banks are among the favourite destinations in the region for the proceeds of corruption.
Singapore, which pays its bureaucrats multi-million dollar salaries, has one of the least corrupt governments in the world according to Transparency International.