You'll be fine with the larger cell, not overkill at all.
Replacing the pump to a two speed will allow you to reduce the Purifier % output even lower, as you will be running your pump longer. Cell life should remain the same though, so there will be no benefit to cell life by going to a two speed pump.
The price difference is even less when you google search the cell prices.

Thanks for considering a salt system for your pool.