I maintain that there is no need for a LS exception, first because it'd be up to your opponent to give you such a large advantage...
You'll need the clause. It won't always be an advantage to the person playing Great City...perhaps not with current cards, but in the future...
Given that Tartaros works without "instead" language (and I think there are others similarly worded that escape me right now), GC should too. The targeting triggers when the card is in play.
Tartaros is actually an instead, because it describes one condition and tells you what to do in place of that. The wording on Great City if it targets the card
doing the exchanging or underdecking is not an instead, but rather just a trigger ("if a card does X, then do Y afterwards" rather than describing an effect and
replacing it "if a card does X, it doesn't do X but does Y instead").
The trigger only works if the cards remains in play. An instead works no matter what happens.
