I add weights behind the piano keys(yamaha g3) for last 15 years. Its around 10% heavier compared to full size grand. Even now days, my finger is strong so as my kids-they all trained on the same grand. When comes to stage, I have no worries to excute my reportories with ease. Of couse, gravity playing is the key point.
Normal downweight is about 50 grams, measured on keytip with pushed sustain pedal. Difference between downweight and upweight should be as small as possible, about 20 grams.
If someone puts additional weights in the keys, regardless on which side, on can reduce or enlarge the downweight. Unfortunately, the more weight, the less responsive will be the key. That means, the key cannot move that fast anymore. That results in a tenacious key feeling.
If I would be used to play on such a heavy action, I would surely also have no worries with a heavy downweight on other pianos. But I would have trouble to play pianissimo. If I am used to push down 60 gram, that is another thing as beeing used to push down 50 gram!
I don't think it is a good idea to play all the time on heavier actions. It will ruin the soft touch, and a soft touch is much more difficult to reach than hard playing, just my opinion.