I assure you, it's not that simple. You either check types at runtime (big performance penalty, aka: C Python) or you don't because a compiler (or JIT) did the leg work.
If you have to double check types at runtime, you've already lost.
But that's why I say, there should be a pypy flag where you tell it to not double check at run time and that you've made sure your annotation is correct.
Now if it isn't correct, you'll probably get a segfault or some nasty error and it'll blow up.
I think that's a fair trade off, and easily doable. Those who want it and respect it get the speedup, those who don't don't lose anything.