Sure but best case 15% faster clearly isn't worth the complexity of a JIT. It really needs to be at least twice as fast. Pypy pretty much achieves that on average.
Of course the counterargument is that they'll improve it and maybe in future it will be 100% faster... But that seems pretty dubious given the progress so far.
It is nice to have the speed, always. But in Python, it would be a mistake to do it at the expense of flexibility. Same for typing: it is great to have it. I use it. But Python should be dynamic and the rest and extension that does not compromise everything else.
If a JIT can make Python 2 or 3x faster, or even 10x for some workloads, that is nice. But the language itself should support same idioms, reflection, dynamic typing, etc at the same level.