New code generation / modification I think we're hitting a point of diminishing returns and they're not going to improve much here
The limitation is fundamentally that they can only be as good as the detail in the specs given, or the test harnesses provided to them. Any detail left out they're going to make up, and hopefully it's what you want (often it's not!). If you make the specs detailed enough so that there's no misunderstanding possible: you've just written code, what we already do today
Code optimization I think they'll get quite a bit better. If you give them GCC it's probable they'll be able to improve upon it
