I happen to be one of them and I'm not saying this just to please you, either.

I've never　written and will never write 出来る as a verb. It isn't found in newspapers, better magazines or legal papers. In nouns, however, it can be written in kanji. 上出来、出来高, etc.

As for 下, it's been explained correctly by the OP. Whether it's written in kanji or not depends on the usage.

コーヒを下さい。

コーヒーを持ってきてください。　

Thanks for contributing your opinion. It's nice to see that I'm not just a crack-smoker. :)

Also, do you know of a reference (book, manual, etc.) that covers this sort of kanji usage? A style guide or something? Obviously I don't expect something written for non-natives here. I'm perfectly fine with something written for a Japanese audience.