At what point would you step up to ferrea hollow stem valves

There are some very light, high PSI spring in the 1.400-1.560 diameters (Assuming a 2.00+ installed height). At one time I thought all high PSI springs were 1.625-1.640" OD. My 511" has 1.600 OD Isky 9998-RAD's and if I re-spring it I'll remove some spring mass there. It already has DLC coated Ti locks and retainers. It will be interesting to see the results.