I wouldn't even consider buying a system that didnt replace the
resonators... I would personally wait until the big companies
such as HKS AND GreDDy come out with their finished products
, then weigh my options..

 

adding in a new resonator for the IS doesnt seem like its bolt on since there are no flanges after the cat, the best gains are when the system is kept at dual the whole time or that merge pipe gets a larger diameter increase with a new resonator to increase flow. Both will require cutting and welding since after the cat the next flange is after the resonator