Are you talking about the pump run time savings? The cell itself won't save anything. But because the T-15 produces more chlorine, you can run the pump less. But if you just change the % setting to a lower value for the T-15, there are no savings.
The T-3 produces about 64% less chlorine than the T-15 so if the % setting is the same, the T-15 run time would be 64% less time. The Superpump 1.5 HP uses about 1500 watts. To determine cost savings, I would need to know your current run time and electricity costs.
So... 10.7 hr @ 2.138 kw is 22.87 kwh for the T3. T15 is 8.77 kwh
So the savings is 14.1 kwh per day Call the season 6 months, or 180 days for simplicity. A total of 2,538 kwh
$600/2538 = .236
Check your electric bill and see how much you pay. If electricity is more than $23.6 per kwh, you'll save. If less, you won't. That number will change depending on how many days per year you actually run the pump and where the water is warm enough that the SWG is producing.
Most likely you CAN save money by switching out the motor to a two speed or variable speed. The SWG doesn't need full flow to work. Nor does your filter. You might also get substantial rebates from the power company that will reduce the expense.
Their method simply uses the motor maximum power rating which is at full load. In most cases, a pump motor will use much less power than the rating. In general, it isn't a very good method of determining actual power usage. The number I quoted is from the Energy Star data set (i.e. measured data).