John,
When I have driven solid wires to what I thought was the max what I have seen is increased spatter that cannot be smoothed out, an uncontrollable arc and puddle, fusion defects, erratic burn off, etc.
Burning back to the tip seems to me more a result of an imbalance between volts and amps as opposed to too much amperage or both. With the self correcting nature of a CV machine once the wire starts burning back, increasing the volts, the current will be reduced, as will the burn off rate, based upon the slope of the machine, and cause the wire to move back towards the puddle, increasing stickout, and thereby reducing voltage and finding balance again unless the balance is too much and the burn back melts on the tip before it can respond.