I think the pay off is pretty far out. I just replaced a 10 with a 13. It is a pretty expensive process. I might save $50 a month on power bills. At that rate I should have it paid off in 90 months or so. You would do better to hoard gas.
I think the pay off looks better when you are in a situation where you have you replace the old unit. Then savings only has to cover the extra cost of the 13 seer.
Also, It is not as simple as looking at the change in RLA to determine the savings. If it never turns off, it can have a RLA of 10 amps and still cost you more per month. Don't assume that since both units are 3.5 tons they will have the same duty cycle. If they dont have the same discharge air temp and flow, they will not have the same duty cycle.