Unless you were born/raised at altitude, living and training at altitude has only marginal benefits for sea-level racing. Living at altitude and TRAINING at sea level (or close), however, has more substantial benefits. Additionally, training for events at altitude is obviously best done AT altitude. Middle distance runners training exclusively at altitude would probably suffer more than long-distance runners, only because it is hard to get proper pacework done at altitude (unless you break it up into shorter repeats).
If you are interested, "Altitude Training and Athletic Performance" by Randall Wilber is a fabulous read geared towards the pragmatic athlete. It summarizes many years of studies on the science and application of altitude training for endurance events.