I also ordered my GT Premium thru Costco. The dealer they referred me to sold the car I ordered at dealer cost, which is about $3000 below MSRP (the car is loaded, $42170 MSPR). There are hardly any 2011 Mustangs around here, and dealers all want MSRP or more. I'd recommend Costco.
2011 GT Coupe - candy red, auto, all the options
Mods - Roush supercharger, FRPP Sport axle-back mufflers, Forgestar CF5 gloss black staggered wheels