I'm not andy, but I'd say that trim pressure is more dependant on the plane you fly.
Plus, the tabs being on control surfaces, they will move the control surface by similar angle for a similar setting, since the forces induced by the airflow on the tab will be proportionally "equaled" to those induced by the airflow on the control surface.
Besides, the pressure for a certain result depends on speed (the faster you fly, the more pressure you need to apply) unless totally assisted controls (I've read that at full speed you could need over 30 lbs of lateral pressure on the stick to make a bf-109 roll, and with such a small cockpit it was really difficult just to deviate it when making a power dive) hydraulically or electrically.
As far as how much forces were generated before and now, there's one thing that remain constant, the control surfaces (especially tail (rudder)) were undersized in WWII, and this on all planes AFAIK
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