No, No, NO,, NO!!!
That's fake difficulty, pure and simple. At the very least, we HAVE to know our dwarves potential. Otherwise you just have to further micromanage 200+ dwarves, and it kills enough time just changing labors to meet your forts needs, let alone the fact they only will do certain jobs.
If dwarves must have a cap (and I'm not in favor of this) at least make the non-capped stat fairly high. And let's not get manual labor tasks (including weapons and such) into this: Anybody should be able to get the highest stat in those (not counting my suggestion of "proven in combat legendary weapondwarf) since there isn't really much to learn, just train.
Another suggestion I thought of: All dwarves can hit the highest non legendary stat in all skils. They arrive with one of their "natural" talents, which have a slight EXP bonus. A few more talents (related, mostly) have a regular EXP gain, and the rest have a weaker version of slow learner. With mental and physical attributes coming soon, this could also tie in with those, where some dwarves are naturally more apt to gain strength, or ingenuity, or comprehension, or whatever, and learn faster at tasks involving those, even against a dwarf with the same stat. For example, two blank title very strong pump operators, one naturally more apt to be strong than the other, would operate at different speeds, with the former being faster.
Anyway, I hope you could understand that rambling. It made more sense in my head, probably.