Not sure if it is possible or no, but what about separating Slice and Spin for Forehand and Backhand?
This will allow creation of a player who has a Very spiny Forehand, but rather Flat Backhand. Or the vice-versa.
The slice when separated will allow for a better control of Forehand-Backhand slice capabilities, and the higher value in that stat will mean more spin on the slice shot, while lower will produce little spin and little "bite" to that shot.
What do you think?