For starters
Structural overload, metal has a memory.
Performance issues, all bets are now off as you cannot use the performance graphs in the manual, theoretically you could extrapolate but what happens if you get it wrong ?
Controllability, you cannot be inside the weight and balance envelope so you may end up outside the bounds of where the aircraft is controllable, how would you know ?
And the attendant risks of overboosting a piston or overtorquing a turbine as a consequence.