I don't understand why you would waste time writing Python code that you're ultimately going to toss overboard and replace with C++ code. Qt's C++ libraries are already cross-platform. Just write your code in C++ to begin with.

There's no need to purchase a commercial license if all you want to do is experiment. Download the LGPL version, which costs nothing. Once you've made a decision, you can buy a commercial license that allows distribution of your executable without releasing your source code. If source code release doesn't bother you, save the money and distribute according to the LGPL.

There is ALWAYS a performance hit when using interpreters versus direct use of compiled code. Period.