Not sure about that. The in-game description of the Shield Generators seems to suggest the heavier your total mass is compared to the SG optimal mass, the less efficient the shield will be.
Edit (add): this might lead to the choice of reducing hull upgrades (mass) to strengthen the shields.
I am sure about that, I know exactly what your saying it gets mentioned all the time but thats why I know it is the way that it is, theres probably been a thread a week since release about does fitted mass work or base.
Easiest way to do a proof of this is check shield modules, if you make your ship too heavy for a class 3, you can still equip a class 3 etc etc. Its been stated by the devs several times as well as the people who ran all the shield calculations initially.
Using ED shipyard for example, you can make an anaconda that has a mass of ~1500t, this is beyond the maximum threshold of quite a few shields (3A max is 413) 4A is 713, 1k for 5A, 1.3k for 6A yet it can fit any of those, because its base mass is 400.
This is the opposite of thrusters, where you cannot fit them if your actual mass goes above their maximum capacity.