In Darkman II, Peyton learns how to extend the 'life' of the synthetic skin from 99 minutes to over 150 minutes. This technology, while apparently so simple Peyton is surprised he never thought of it in the previous film, is never seen again.
Dr. Thorne is (or at least claims to be) one of the doctors who cut off Peyton's pain receptors, leading to his enhanced strength. Yet, despite familiarity with this technique and its side effects (familiar enough to reverse it), she cannot duplicate the altered adrenaline in Rooker's thugs simply by performing the procedure on them (or at least some poor sap they don't have to hunt down first).