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If you are doing the install yourself, then it's only a $75 dollar experiment. Those springs will wear out your V6 coupe shocks and struts and you may get a bouncy ride sooner than later and since yours is an 05, your shocks and struts are already well used.
Think of it like this, getting those springs is like wearing a pair of women's pants if you are a guy. Sure, they are pants, but they weren't designed for a guy and therefore will not perform optimally.
They likely have a stiffer rating that you would want, since its meant for a heavier GT convertible and you have a v6 coupe. Whatever the estimated lowering amount is, I'd consider it to actually drop it a little less.
Of youre planning a swap then get em and hang onto em until you get that weight on the front end...it wont hurt anything to just hang onto em if the price is good...heck offer him 50 bucks.