The Summit converter was probably described as "1700-2100 stall" based on the characteristics in the most common application: Chevy 350 small block, which has far more torque at that speed than your 250. Stall speed will always be proportionate to the square root of torque.
This means that without anything wrong the stall speed will be significantly less than advertised, and your idle speed drop into gear will be greater.
The converter doesn't release, the only internal change occurs when the car rolls out.
500 does sound high, but I can't see a specific remedy.
Perhaps someone here has an idea?