I am still thinking which brand should I choose HKS Hiper or 5 Zigen?? HKS is the catback system and cost my $1400 CDN but a 5 zigen is only rear section but is cheaper $500 CDN. However, I think the factory exhaust is the reason to make the difference of HP between GS and IS (10 HP), so should I choose hiper?? Than HP gain!!! HURRY!!!