I'm not certain about discus throwers but I have done quite a bit of research on the best U.S. shot putters and I would suspect that much of the same things I've found for the rotational SP can apply to the DT. Here's what I've found with regards to knee angle in regards to U.S.'s elite shot putters (top 12):
*There doesn't appear to be any advantage with a particular knee angle (in either leg) at any point of support prior to rear foot touchdown.
*The knee joint of the R leg (in a R hand thrower) should be relatively extended and far away from the rotational axis of the body as you turn on the left foot towards the front of the ring. This will increase angular momentum as you enter the center of the ring and will increase angular velocity as it gets closer to the body. I am certain this is beneficial for discus throwers.
*At rear foot touchdown, their appears to be an advantage to greater flexion in the rear knee and their doesn't appear to be any advantageous angle in the front knee.
*In elite shot putters, I've noticed a counter-intuitive trend that indicates that more rear knee flexion is better at the point of release. For a possible explanation for this see my article on the critical factors for the shot put.
*There doesn't appear to be an advantageous front knee angle at release in the shot put but in the discus Dr. Bing Yu (who does what I do but for the discus throwers) has found that greater extension of the front leg is very important.