An A320 burns about a half pound of fuel per second when taxiing. [1] So it seems plausible that the amount of weight you save on fuel could go a significant part of the way toward offsetting the weight penalty from carrying the extra motors.
I suppose if you're pushing the envelope precisely, and push back overweight, count on the taxi burn to get you down to max take-off weight, and plan to land with the legal minimum fuel reserve, then yeah, you'd still pay the full weight penalty. But I'm guessing that most airlines, as a policy, anticipate a range of possible taxi times in calculating their weight and fuel budgets, so significant weight savings should be realized.
I suppose if you're pushing the envelope precisely, and push back overweight, count on the taxi burn to get you down to max take-off weight, and plan to land with the legal minimum fuel reserve, then yeah, you'd still pay the full weight penalty. But I'm guessing that most airlines, as a policy, anticipate a range of possible taxi times in calculating their weight and fuel budgets, so significant weight savings should be realized.
[1] http://www.mit.edu/~hamsa/pubs/KhadilkarBalakrishnanGNC2011....