Currently slope only seem to affect maximum speed. Ideally, moving uphill at any given speed mode (and downhill to a lesser degree) would also cause greater fatigue than moving over flat ground.
Some approximate empirical maximum slopes:
Stay prone:
A2 35°, A3 32°
Stand without sliding downhill:
A2 40°, A3 40°
Run up:
A2 20°, A3 35°
Sprint up:
A2 10°, A3 32°
Run down:
A2 -30°, A3 -35°
Sprint down:
A2 -25°, A3 -35°