[Ulf]

In 25-28 c water the water in the suit will probably soon reach a temp of 30 c or something like that. I would assume that is way to warm to get any decent vasoconstriction. Maybe that could explain the difference in time.

When I did Sebastien Murat’s course he made us do a test to see how strong our DR was. The test went as follows, you start with checking your heart rate, then you do a FRC breath hold (without breath-up or warm-up) with your face in really cold water and check your heart rate when you get contractions. Maybe you could try that to how strong your DR is.
If your heart rate drops about 50 % you have a decent DR and if it drops 70 % you have a really good DR. If I remember correctly, he considered anything less than 50 % pretty crappy.