Is a high performance interpreter to Python realistic without create incompatibility in the language with the regular interpreter? Most similar language has similar performance (or slower)?
There are a few well-known shortcomings to the Python runtime that might be fixable without hurting compatilibility.
However, compatibility is somewhat of a red herring. The new runtime would be targetted to mobile devices which are incompatible to desktop hardware by definition (no keyboard, no mouse, touch screens, lower memory, slower CPUs, battery considerations, new APIs for geo-location, multi-touch, etc etc etc). Desktop apps would need to be ported anyway, so this is not that much of a problem.
Yeah, it's ugly how Java is released as free software but the TCK is not.
Now, the real question is what would happen if you forked Java and called your form Jabba (the Hutt), without making any compatibility claims. Would Oracle still go after you?
No, but you wouldn't be able to sell your products either. Virtually all serious businesses require some proof of compatibility. Without that, they won't touch your product with a 10ft pole.
However. Lately, JBoss has been slow in getting their JavaEE certification, but still released new versions of JBossAS. Granted, they have already made a name for themselves and can afford to skip some certifications, but this could turn into a trend setter.
Personally, I'd like to see Java turn into an ISO standard, but such requests seem to be deleted on Oracle's site.