If the GTS's V6 is designed for premium (which if i remember correctly, it is), you should stick with that. Basically there's a certain quality level in premium fuel, and if you drop down, you'll start wearing the fuel system out. also, the engine's made to run on premium so you wont get as good of gas mileage and power. Stepping up in octane is always good, but avoid stepping down if possible.
They recomend running premium because of high compression. It keeps the motor from predetonation or "knocking" or "Pinging". Too much knocking will end up wearing the motor out. As far as gas miliage.. I have never noticed or heard of a significant difference between regular or premium. So its good to go higher octane.
10 gallons at $3.30 for regular = $33
10 gallons at $3.50 for premium = $35
$2 every fill up isnt bad for a relyable motor.
Generally speaking the octane ratings are higher than advertised. That is a 'minimum' rating after all. However, that being said the comments on pre-detonation wearing out the engine or damaging the valve train are true.
Another trouble may be incomplete combustion, which can cause run-on after you turn off the engine (less common but not unheard of in FI engine). Also wears out the engine because the revs drop way down and cylinders fire unbalanced causeing uneven strain on the rotating assembly.
My opinion would be not to drop below 89 octane in your car requests 91. Granted I would recommend the 91 just because they generally have more ditergants in them and keep your valves from sticking in the long run.