CMB is a neat destination and is much safer today then a year ago-although not perfect. It is nice to hear about the fast track cards that SQ is giving out since the lines can get long at certain times. As for an immediate turn I have never done this, but it sounds like a few have and I can picture this going as smoothly as mentioned. If you do clear customs keep in mind that there is a departure tax that will need to be paid.

Just for my interest-why would you do a turn around to DEL, KHI or CMB or anywhere else in that region from HKG?

It doesn't appear to be a mileage run per se since LAX or JFK would fit that title much better.