MOF is ultimate control over temasick and GIC. Once pap loses 2/3 majority, you can expect the perm sec and other senior civil service staff will become more neutral and accountable. the opposition MPs will have more inside info to ask really tough questions.
So your assumption is Perm secs and senior civil servants are robots who only do their jobs without fear or favours?
Also, the relationship between the civil service and gahmen linked companies are blurred for a reason. And the people up there play that to their advantage. Even if the next minister of finance and the next Ceo of Temasek don't share the same bed anymore, I'm sure they'll still be good buddies.
The only way things will change is when a big player within the system stages a takeover. Or when GLCs become so badly run they are sold off to become "truly" privatised. No rational person will give up a position of wealth and power. And while all this happens, I don't see life getting better for normal salt of the earth singaporeans.
The only solution for this country is to invent a time machine so that sinkies can go back to 2006 GE and not give PM Lee a "strong mandate".