The Movie: Aldrich Killian breathes fire. Yep.
Real Life: The whole idea behind Extremis seems to be about manipulating bodily processes that already exist. We already have genetic mechanisms that regulate healing, strength, and body temperature: what if we could amp up the juice on those mechanisms, right? Okay, fair enough in theory. But fire breathing? When in the history of human evolution did we develop the capacity to barf fire? What gene could possibly have been revved up to produce this adaptation? Did we interbreed with dragons at some point? We're not saying we don't want to have atavistic fire breathing genes. We're just saying, "holy cow, that's freaking crazy." And makes no sense.