Well statistically, because of the fact that there's very few SRT6's on the road, it was probably a modified N/A Limited or Base. Did it have the fixed rear spoiler or the smaller retractable one?

MRX means basically nothing when it comes to the Xfire. There's no nomenclature that exists for this car that includes "MRX". As you probably already deduced, it's probably just some finishing touch on some person's modded-out Xfire.