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I hear about Steeda mounts but they are not a direct fit, you need to do some modifications, apparently getting parts from other oem mounts, to fit them on a '11.
FYI, Maximum Motorsports makes a CC plate specifically for 2011+ that uses the 2011 isolator/spring perch (the part in question). Not sure on spring fitment though, I guess you'd have to call them.
If you can't get ahold of the GT500 mounts, I'd seriously just consider going aftermarket, even if it means just going to the dealership and getting a pair of 2010 strut mounts so you can scavenge the isolators. Its worth it, IMHO, to get a much better strut mount with good adjustability.