The twin turbo is the way to go... the entire engine is already set up for turbocharging, meaning that you have to do less to it to up the boost. There are many more tuning options available for the twin turbo, whose engine is shared with the highly-moddable Supra, than there is with the V8, which is shared with the boulevard-cruising Lexus.
If you were to turbo a V8 you'd have to get new pistons, connecting rods and possibly a new crank depending on how much boost you were throwing at it; you're talking many thousands of dollars there... the amount of money you'd have poured into the V8 you could have gotten a couple hundred more horses out of the V6!
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