So, this is useless if you can't play to cast it? I don't think it will be a feel good card in limited really. A 5/5 flying in artifact is strong yes, but does not warrant it being a 0/0 by default.

It will be feel bad when you can only drop it as a 1/1 or 2/2.

­Skyreach Manta had a similar problem, except getting it to a 2/2 was trivial.

Then how about a slight change?

Steamship
Artifact creature - construct
2/2
Steam powered - ~ enters the battlefield with an additional +1/+1 counter for each spent to cast it.
­ : It X is greater than or equal to ~'s power, it gains flying until end of turn.

I don't think that design works either. Having a 5/5 for 3 at best is ridiculous.

I don't think this design fits for a pure artifact. As a mono-hybrid maybe. (Something like would had worked well. at uncommon maybe?)