Sure, you can get a blister immediately when switching shoes which is probably due to the new shoe fitting differently but you don't get a "dodgy piriformis" all of a sudden. It's a pretty tough little muscle that can withstand months or even years of abuse before it complains. Even if the new GTS was exactly the same as the previous version, just the extra effort it takes the foot/leg to flex in a brand new (stiffer) shoe could be the straw that breaks the camel's back, but there must have been an underlying problem already present in the first place that has built up over time.