I suppose there is no absolute answer as Rusty didn't say what type of thickness resize and either didn't say what sort of volume reduction we were looking at or I can't remember. Nevertheless the message is to encourage us to consider all aspects of a custom order not just volume e.g if we are thinking of more volume to help stop us from bogging then think planing area instead - an increase in planing area is what is going to keep us going.So is that a mild drop in thickness at with proportional rail thickness drop makes for a bigger increase in sensitivity than a mild drop in width? In which case I would be inclined to agree with you.
Or do you mean rails the same but drop in thickness at stringer only (flatter deck) makes for a bigger increase in sensitivity? In which case I would be of the opposite opinion.
Rusty did say something like the "key to volume is thickness", so I suppose we can use the two interchangeably.
Just out of interest, on my toy step up a 1.5 litre reduction in volume can be obtained with a 1/8" drop in thickness this does a very small proportional rail drop height of about 3/32" inch. So we would be looking at a bigger litre reduction than that for something significant.