I can't speak for everyone but I personally think that the life span is dependant on various factors just as any cisperson. Not to mention for post-ops I would imagine some of the dysphoria stress would go away making them a bit more healthy or even the cases where for some people transitioning is a life or death situation.
Who do you think will have the longer life span? A post-op trans who exercises and eats well and take care of their bodies or a biological cisperson who is always at high stress, morbidly obese, constantly smokes, drinks, and do drugs etc. on a daily basis?