Not so sure whether SAF is flexible on running shoes for trainee. Maybe for recruit doing BMT. They might not have a choice. After my BMT way back in early 2000. We were allowed to wear any running shoes for our run.
When a person is into running. He won't bother about what is the saf shoes being issued to him. He just get his own prefered running shoe.
For a US size 9. As long the shoe is more than 280g. It is considered heavy. Between 260 to 280g is just average. 240 to 260g not bad. 220 to 240 quite light. Below 220g. Super Very light and better use for sprinting fast pace like 2.4km under 7 mins timing. U know army like to go for camp run and that why they want a all rounder shoe. So it tend to be on a heavier side.