Personally I don't see the point in this. (at least for attack/dc) if you want a +10/-10 trade off you can just take +5/-5 and use the power attack feat all the time to bump it up further.
That said 20 points of impervious toughness would just be annoying, so I will vote yes. Either to +5/-5 limits or 50% of PL rounder down.