Well its not really just GTR tax, Akrapovic charges high for all their exhausts. Its about establishing a premium brand, and just like all premium brands are they overpriced? Yes. However you get the best quality, the exclusivity and IMO best sound too.
Worth the extra outlay? Well if the GTR is your dream car and you want the very best parts for it, yeah for me it is. In the process of sourcing one now.