SWCG life is a function of cell "ON-time". Our experience here is that most SWCG cells barely last to the end of their warranty, so we recommend over-sizing, rather than under-sizing. The reason is that if your SWCG is 2x the size you need, the ON-time, per pool season will be 1/2 of what it would have otherwise been . . . and the cell will last 2x as long (in years of pool operation, not in hours of cell ON-time).

By going with an AG SWCG, you'll be under-sizing rather than over-sizing, and this will result in premature failure. Most likely, the warranty will have a "cannot be used on IG pools" disclaimer, so you'll have no recourse when failure occurs.

BUT, the AG unit will produce just as much chlorine (while it still works) as it would have installed on an AG pool.

There's one other point to consider: installing an SWCG on an existing pool can result in problems. The increased salt DOES increase corrosion, and your pool may not have been built with that in mind. Some regions have more problem with salt than other, mostly due to dry climates resulting in the salt not being rinsed off by rain. You elected not to include your location, so I can't tell if that applies to you or not.