Well the SVT system is pretty darn good already, any gains with aftermarket catbacks are minimal.
There is a post around here about someone who took out their rear lunch box resonator of their SVTF, and they liked the sound that it made after that. That would be a cheap and viable alternative so you can spend the un-used money on other, more significant mods.
The HKS system will be like any other system for the SVTF, with one difference. Its design is to go underneath the rear suspension unstead of routing through it. This makes your exhaust that much closer to the ground, so if you decide to drop your car, you'll have to be careful.
Good luck on your exhaust ventures
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