Fugetaboutit. With spherical front control arm bushings you'll have enough to worry about just to get enough camber in the front. Caster at this level has very little to do with the total grip. Leave it stock.
Here's the keys to handling with a 240Z: Properly managed Hoosiers, shocks, front to rear balance controlled with spring not big sway bars, shocks, the right camber, shocks, driver training with DAS, bump travel, and shocks. Then there's the key to lap times: getting on the gas pedal earlier than anybody else because of the aforementioned handling key. Oh, and the gas pedal should be connected to a Sunbelt engine.
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katman