You say low nitrites, nitrates, pH and ammonia, but that really does not tell us enough. Can you give us specific readings from the tank ?
You can lower the water level so that water from the filter splashes, and add another air stone, all of which will increase gas exchange, which will assist in bettering the oxygen level, but very warm water can't hold as much as it can at cooler temperatures, it's simply the way it works. I would drop the temp to 80-82 at most, at least until the fish are comfortable and appear to breathing normally for a week or so.
It appears to be the case that they may breed better at the higher end of their temperature range, and even grow faster, but if they can't breathe well that's not going to be much of an issue.
And I agree, cories are not warm water fishes. They prefer it much cooler and are going to suffer ongoing stress, which will shorten their lives, in a tank kept at 80 or higher. They can tolerate it for short periods, but it's not what they want or do best in.
Mine are doing very well at 72-74, and it has been colder than that at times. Temperature control is difficult here as I have no control over the heating in my apartment, and can't afford a system that uses a thermostat for the fans.