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In theory you're better off with a separate throttle and intake runner for each cylinder. That provides the straightest, most direct flow path with the least turbulence.
Beyond the cost of the TBs themselves, though, you need to factor in the cost of good intake manifolds (R-Sport if you can find a set, or Misab or TWM or similar) and the cost of matching them to the ports and TBs -- they don't come finished, just rough castings. You can do this yourself if you're enterprising -- what's desired is a totally conical shape tapering from the TBs to the ports.
Then add cost for linkage, horns and filtration.
Injectors in the head are favorable for low- mid-rpm performance, as the fuel is atomized close to the chambers and is likely to arrive there well-suspended in the air. At high rpms / airflow / fuel rates, the curtain of spray is disruptive of the airflow -- injectors in the TBs is the favored format.
That's the theory... in practice, the D-jet manifold isn't that bad. The runners are a good tuned length and the single throttle is generous in size. It also fits on the car without reshaping any sheet metal. I don't have any data on how much improvement individual TBs would make over this (and neither do others I've asked).
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