Interesting question ... I've been training for some uphill mountain races. The group that I have been running with has come to the conclusion that we should judge our "pace" by rate of climb ... feet/minute, feet/hour rather than min/mile miles/hour. Obviously pace per mile is significantly different for a flat 10 mile run vs a 10 mile run that involves 3,000ft of climbing. Despite the fact that you might only be running at 9:00 min / mile ... you have to factor in the rate of ascent.
I think that you could determine the answer to the question with the aid of an HRM ... where you would run a measured distance on the flats, maintaining a set HR, then run the same distance on a known incline, for the same distance, while maintaining the same HR, ... then doing the same for other inclines. I would say that a treadmill would be good tools, but treadmills are notoriously incorrect with regard to incline and especially distance.