Wikipedia defines transhumanism as:

Transhumanism, abbreviated as H+ or h+, is an international intellectual and cultural movement that affirms the possibility and desirability of fundamentally transforming the human condition by developing and making widely available technologies to eliminate aging and to greatly enhance human intellectual, physical, and psychological capacities.

So what I mean by "the feasibility of transhumanism" is just the "possibility" half of that definition, setting aside the desirability.

Even granting all that, I suppose you can still quibble about semantics, but I ran through several possible labels for what I had in mind and that seemed the best choice.