hello technerd,
Speaking for me, normal (non-S.Art) and S.Art voices have similar loudness characteristic and so I play them the same way. Of course, S.Art voices have articulation (expression) capability, so playing technique depends on voice we choose -but this has nothing to do with loudness. That is, for example, I play non-S.Art piano the same way as an S.Art piano -means, I don't need to use higher velocity (force) on S.Art voices.
Saying all that, I'm surprised that you say you have trouble with S.Art voices
only.
Btw. differences in Touch sensitivity (Normal, Soft1, etc) are quite subtle and so when changing from one to another, no big change should be expected.
Another thought comes to my mind... is this your first keyboard? Or first keyboard that has touch sensitive keybed? If that's the case, then the
problem is most probably your imagination.. or to put it differently, it doesn't behave as you expected. When I bough my first keyboard, I also had impression that the thing is just too quiet and voices were kinda flat.. not crisp.. and so I simply turned touch sensitivity off -which is not a solution, of course.
Obviously, when touch sensitivity is used, then you need to turn volume knob a bit louder (when practicing, I usually have it at about 10-11 o'clock position). And no, you don't to hammer the keys

Bogdan