Understand your dilemma
I know your dilemma. I just dropped my 2013 GT premium off for a warranty with a differential leak. The dealership got in 2 2015s so naturally I climbed underneath and started asking questions. While I was there, I figured I would get a trade in value which seemed fair once I return everything to stock. I go to road courses so the appeal to me is a little more with the IRS. Not a fan of the performance package coming with the 15' brembos since this renders my 18' track wheels useless. I did enjoy driving it and the track mode setting on the S550 is different. The steering wheel seems smaller too.
FR Sway bars, BMR adj. panhard/brace, FR radiator, 302 Oil cooler, Strut tower, SHR Louvers, Roush Upper/Lower Grilles, Roush Axle-backs, Hurst Short Throw, Corbeau FX1 Seats, Corbeau Harness bar/ 5pt. Harnesses, Steeda X-brace, DBA 4000 rotors, Hawk DTC pads, GT500 18x9.5 with Toyo RA-1 for the track, Staggered 2010 GT500 19's for the street, Brembo brake upgrade, Cooling ducts.