There are 3 categories for SAF Officers - A for normal entry/ diploma holders, B for graduates and merit award scholars and finally, C for SAFOS and Presidential scholars.
Don't know much about the first two grades but for B, the max you can go is COL. For C, they are promoted one grade every 2 years and reach super-scale grade when they hit LTC rank (which is ~$20K a month). That's when they start to shortlist potential ministers and accompany their PAP mentors on overseas trips.
Ministerial caliber scholars are extracted before they reach staff position. The norm is that if you hit COS, that means you are not ministerial caliber and is sent to Stat Boards or retired after serving your term. So if you read about some "leaders" who are chief of this and that in SAF; they have been vetted and found wanting.