I have no experience with any of the above, although I've researched a lot and plan to add a Harrop system in my car in the next couple of years.
Harrop might be the most widely used in the LS community. Mast Motorsports makes engines that use the Harrop, DSE uses it in their White Monster 5th gen Camaro. But I think any of the above vendors could be successful as long as the engine and desired performance parameters are spelled out clearly in advance, the system custom configured to requirements, and good EFI tuning available (ITBs present quite different challenges to the ECU in low throttle situations).
Biggest advantage of a true ITB in a streetable application is the ability to use a dramatically larger cam (specifically, larger in the sense of more overlap) while still maintaining idle quality and lower RPM drivability. That's because the close proximity of the throttle blades to the intake valves creates a much smaller total intake volume that is under vacuum, and therefore a smaller volume of exhaust reversion during overlap.
Also, an ITB is typically easy to "tune" for optimal effective runner length and thus maximum peak torque RPM.
The biggest downside to ITB is that creating an effective filtered cold air system often robs a good deal of power. It's not an issue of "flow" per se, it's that most air supply manifolds partly suppress the finite amplitude waves in the intake runners - those pressure waves that help pack the cylinders before the intake valve closes. Creating custom air supply manifolds with large volume is important to minimizing this loss.
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