Question: I was wondering whether the rotational speed of a discus has any influence on the flight of the discus. Would slowing the rotation or speeding it up change the trajectory in any way or would the flight simply become unstable when slowing down?

Answer: 
Allegedly the rotation has two effects. I say "allegedly" because although I was told this in physics lectures at university I'm not sure if anyone has ever rigorously proved it.

Anyhow, with the disclaimer behind me, the first effect is that the angular momentum stabilises the angle of the discus as it travels through the air. That allows the angle of attack to be maintained at the optimum value and hence increases the lift and therefore the range. The second effect is that a high rotational speed makes the boundary layer turbulent, and this decreases aerodynamic drag and once again increases the range.

Given this I guess higher rotational speed is better, though presumably an athlete is limited to how high a rotational speed they can generate without compromising the speed they can throw at.
Given the physical conditions, this seems like an appropriate explanation: The faster the discus rotates, the more violently and quickly it displaces the air around it. Now the absence or scarcity of air causes a reduction in air friction or viscosity around the discus and this allows it to move onward in the direction of propulsion; now that depends on what angle the athlete projects it. After a certain distance there begins a constant deceleration of rotational speed because at some point, the air friction starts overpowering the rotation and this results in the discus entering the second half of its trajectory, i.e., moves downward along a curved path. I hope that answers your question.