Thanks, Mark! This was driving me crazy.
My fingers are so used to using ESC that reaching over to F5 felt really awkward. Not only that, but making the choice to either flip the F keys back to their function key equivalents (thereby losing audio and brightness controls) or being forced to hold down the fn key before striking F5 was a lose-lose proposition for me, so I did a bit of digging and found a little gem on Stack Exchange http://apple.stackexchange.com/questions/254306/enabling-word-completion-by-esc-key:
In addition to F5 for code completion, you can also use OPTION-ESC. I tried it and it works like a charm!
The Stack Exchange asker was looking for a solution to return to pure ESC - but no luck there. Failing that, IMHO, falling back to OPTION-ESC is a much more comfortable and natural compromise than F5.