The reactor is believable somehow.
But Tony flying any faster than 50MPH just strains the suspension of disbelief...(I wonder what makes 50MPH the magic number).
An armour that is skin tight(II, III, IV, VI, VII, and especially the smaller MKV), screwed together but water/airtight, can protect the user from any impact no matter how large, can fly using a magical thruster device that accepts electricity in one end from a reactor, and pumps out raw thrust on the other end, and it's the AERODYNAMICS of a larger suit(Which, ironically, has a LOT more internal area for larger thrusters) that isn't believable.
So. We have a guy here who watches sci-fi movies but can't enjoy them due to them being unrealistic.
I think we have a physics professor in our midst fellas...
Either that, or a maths teacher.