If the manual says it is an optional element replacement, it should still pass all certifications if you substituted a bigger set of elements.
If your house has enough vertical drop to accommodate one, a long-term savings may be to install a waste water heat recovery system. This will make the existing WH look bigger, as it preheats the incoming water from that going down the drain. It's discussed here often...just search on those terms. Depending on your utility rates, and the amount of hot water you use, would determine how quickly it might start saving money after the purchase and installation.