While I can see the logic in your 1.2%, I believe that the addition of drafting in the cycle stage has made the running stage more reliable, because the athletes in a cycling pack can conserve their energy for the run. I would therefore argue that a smaller improvement in performance is now worthwhile, probably a little less than 1% in run speed.

You have not dealt with the women's triathlon. The smallest worthwhile improvement is likely to be greater for women than for men, given that the men's competition is generally closer (that is, the top ten finish closer together). The International Triathlon Union takes this difference between sexes into account by setting different cut-offs for triathletes to earn competition points: 5% of the men's winning time is the cut-off for men, but for women it is 8% of the women's winning time.