I think what you are referring to is the best angle of climb speed. Green Dot is the minimum drag speed and will therefore give you the best angle. What I am after is the best rate. For an average 320/321 at mid altitudes and average weights it should be in the region of 275 Knots or so.
If you were only at Green Dot at FL 300, you would be using quite a bit less than maximum thrust. In the event of an RA you would have to apply TOGA and maintain VLS as a minimum. But, yes, you would be unlikely to achieve 1500 FPM for any decent length of time.