Macritchie is a lot tougher than Bedok, there's this club MR-25 that does 5k monthly time trials at Macritchie.
http://mr25.org.sg/
The national runner was only able to run 17.07 min for 5k, when his actual 5k time is around mid 15 min.
Bedok reservoir if I am not wrong, usually add about 30s to 5k time, with the sandy terrain. Not too bad, but enough to feel the fatigue setting in earlier, compared to running on road.
Yes, this race is actually very competitive. Top 50 times last year was under mid 17 min for the 4.8km. And that is equivalent to 5km on road. The top 50 consist of a lot of foreign runners, but still have a number of fast local runners.
Not all of these runners transit into long distance though.