I think if you can run comfortably at 15mm, your form is likely to be very good. The trick is not changing your form when you gain speed. Like anything else, try running intervals, fewer and shorter than what you think you can. Lean forward a little farther at the ankle - NOT at the waist. Open your stride up a little longer behind you - NOT in front. You should feel it in your obliques.

Didn't you say you used to run track when you were younger? I'm betting you find it again sooner than later.