Hi Sam.
As the C of G is moved further forward, the stick force, and control deflection to maintain altitude in a steep turn, will increase, until it reaches a point where either the force becomes too great for the pilot to maintain, or the elevator reaches the limit of it's travel. In either case, the aircraft will be unable to maintain altitude.
As the C of G is moved further aft, the stick force/deflection will reduce until a point is reached where the aircraft becomes unstable, pitches uncontrollably into the turn, and stalls/spins, a condition from which it may be unable to recover.
As Dobbin mentioned earlier, the C of P moves forward in a steep turn, so there will be a rearward C of G position where the aircraft is marginally stable in level flight, but becomes unstable as a steep turn is entered.
Hope that helps.
MJ
Ps. As 3 point says, Please don't try either of these at home!