With the kind of production numbers the S197 has seen, combined with as long as modern cars are kept these days it's not very likely an ordinary '05-'09 GT will be much of a collectable where an original exhaust system will hold much value, at least not in next 10 or 20 years. In 40 years who knows, I know a couple guys who'd give their left nut for an original, unbent, unrusted exhaust system from a '69 or '70, but even with that example there's aftermarket duplicates of the original that's perfectly acceptable for use in a restoration which limits the actual $ value originals will have. So, unless you feel like hoarding those pipes for the next 50 years (and keeping them in perfect condition while you're at it), I think you're safe to unload them. Try craigslist, I'd bet there's plenty of V6 owners willing to take them off your hands for their dual exhaust conversion.