Why would you need to change what Python does (i.e. the semantics of the language)? All I said was that you would find a subset of Python that is useful for your project, then put together a Python compiler that is very good at optimizing that subset. This is not at all unprecedented: Microsoft does this with the C compiler used for Excel and ITA software did this with Lisp when they modified CMUCL's code generation routines (CMUCL and the SBCL fork are particularly interesting here, as they expose their code generation modules to the programmer, reducing the effort needed to hack your compiler).

The point is that these things require a substantial time commitment, which is what I said in the first place. If you have unlimited time to work on your project, you can use whatever language you want -- your time is unlimited -- and still meet all functional and performance requirements.

Of course, time is rarely unlimited. Usually you need to meet a deadline, and that means that you need to prioritize, and your priorities will guide your language choice. In my experience, functional requirements are typically higher-priority than performance (with a reasonable margin). Getting the right answer slowly is usually better than getting the wrong answer quickly; getting all features to work is usually more important than producing a fast system that implements only half the features.