Eh, big stretch we knew their young grew so fast when there is so little fossil evidence to indicate as such. I mean, there's very few complete t tex fossil finds, and even fewer that are juvenile. You'd likely have more success throwing darts at a dart board than predicting anything based on what we've found. As for being optimised for running, were still not sure how that worked. I mean, a lot of big cats are optimised for running, but very few run for sustained periods. See lions, cheetahs, tigers, etc.
Yeah, they could enter a torpid state, but that doesn't mean they have to be vulnerable. We have many large reptiles now that do the same and aren't at a significant risk of predation, if they were they wouldn't do it. Also, we have some mammals that do the same and they aren't predated to the point that they stop doing it.