Just to clarify, its not that the LTH's or HFC's are bad its that Stillen didn't design it for those applications. On normal applications improving exhaust flow usually leads to gains on an S/C setup. Hell on my 4 banger S/C, a decat and headers will yield significant positive gains. This was my biggest annoyance with the z34 S/C platforms. You are forced to keep your stock cats which are EXTREMELY RESTRICTIVE and leave power on the table.
Other reasons why improved exhaust doesn't seem to help S/C applications is because VVEL will not compensate properly for the improved flow.
OP since you are in Cali, its probably best to leave the stock exhaust hardware on, are you going CARB certified on the Stillen S/C? If so be wary of the canned tune.
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2009 Carbon Fiber/Red 370z aka Trish: Sports Package, Meisterschaft full catback, Fast Intentions Longtube Headers, Injen Long Tube Intakes, UpRev Tune: Sold 
2008 Lotus Exige S Club Racer aka Bella, always leaves me smiling
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