Comparing a stock 6 to a stock 8, I guess that depends on how much it'll cost you to get to what the 8 comes with. FBO and cams gets you close to the same power but you'll still have a bunch less torque (not sure how much E85 helps; don't have E85 around here); the SS also has wider wheels/tires and Brembos by default.
Is a 6 really only worth $6k? Up in Canerda I'm seeing used ones for $14k to $24k, and SS's going for $26k - $34k, so $20k'ish vs. $30k'ish, or $10k/50% difference.
But the whole "why mod a 6 when you can just buy an 8" argument is stupid, 'cause let's be honest, if anyone is/was willing to mod their 6, they'd be spending the same amount/doing the same mods to their 8 anyway, had they bought one
Guess it all depends on how fast you want to go; mine can do 0 - 60 in under 6 seconds no problem and that's good enough for me, plus I get a lot better mileage than an 8.
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Overkill/Self Tuned; Mace Camshafts; K&N Typhoon Cold Air Intake; Ported 80mm Throttle Body/Intake Manifolds/Manifold Spacer; Solo Performance High-Flow Cats, Cat-back Exhaust; Vitesse Motorsports Throttle Controller; Elite Engineering E2 Catch Can; BC Racing BR Coilovers; JPSS Delrin Radius Rod Bushing Inserts, Sway Bars; Pegasus Aluminum Rear Cradle Bushings, Camber/Caster Plates; Z/28 Toe Links, Trailing Arms, Upper Control Arm Bushings, Rear Shock Mounts