I don't see the value of those sedans sticking around very long regardless of transmission, sure there's always going to be a preference for manual amongst some and that would command a higher price, I doubt that price will be much of a premium soon enough though, at the end of the day the cars getting on and it won't be long till the shape is plentiful and cheap on Trademe.
I personally don't put much in the added value of recent maintenance items on a vehicle like this, maintenance is maintenance is is required anyway, sure it's great to know this and this is new, but soon enough that and that will require attention also. I don't think time is on your side when comparing the now and then of the past, I also don't think cosmetic upgrades will help stave off the pace of devaluation either.
I'd expect the coupe's to hold their value because they're a much more desirable car. It's practically already a classic shape, just needs the 2.8 engine and manual trans..