You are mistaken and need to reread the information in question. If we are both talking about the same study showing no suppression and 3,5 showing 75%, then take a closer look. The studies references GH and not thyro activity. It references the suppression of TSH.
The GH response is a function of the 'thyromimetic' activity of the compound. The study in question showed that 3,3 did not restore GH function and did not affect TSH levels, which would be expected from an inactive compound. T3 and 3,5-T2 both demonstrated thyromimetic activity (restoration of GH and suppression of TSH) because these compounds are active.
This study did not look at metabolic rate, but from the literature, 3,3 has scant evidence in this regard, whereas 3,5 has a strong consensus. In fact, the write up for this product uses studies supporting 3,5-T2 and not 3,3.
The irrational fear of TSH suppression seems to have driven people to a compound that has little science to support its use.