Werebat
Explorer
It simply turns the feat into a "half feat", with the other half being a +1 stat - consistent with a bunch of other feats. The -5/+5 is another option I guess. Personally if you want a -5/+5 option, I suggest making that a "called shot", and add it to the list of actions (ie anyone can do it without a feat).
That's an interesting idea.