Curb weight is the car in drivable form. All fluids full.
Also, pretty much all aftermarket exhaust components are going to be lighter than their OEM counterparts. I'm not so sure about the Stillen headers, because they are of a similar design to the oem headers, but everything else will be lighter.
Looking at the mod list in your sig, there are several items that would have added the weight back on. You should be happy that you are only coming in 150LBS heaver than stock. The extra HP from the SC kit more than makes up for it.
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Competition Clutch|Fast Intentions|Maxbore|NST|Setrab|Stillen|Tomei|Uprev| Z1
Avant Garde|SPC|Stance|Whiteline
Aerojacket|Seibon|Tanabe|TRS|TWM
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