I suppose I'll put my .02 in here.
From what I theorize, the spacers serve to lengthen the intake runner length and also to allow slightly more opportunity for fuel atomization. Engines are designed a certain and certain things are designed to match other components. For example, the intake manifold is designed to work with the valvetrain to place the torque curve in the desired location. If I am thinking correctly, lengthening the intake runners (which is kind of what a spacer does) has the potential (depending on the design of other engine components of course) to place the torque curve lower in the RPM range. This however, may/will sacrifice higher end power because horsepower is a product of torque and rpm. If peak horsepower was increased by a spacer I would assume that it was due to more effective atomization. If everything else was being done effectively (ie: exhaust clearing the combustion chamber, correct timing, adequate air delivery) that the throttle body spacer would be useless. Therefore, get a throttle body spacer only if you are looking to shift your powerband slightly lower, as the intake runner length is already designed to work most effectivly with the valve timing and other important processes.
I'm no automotive engineer, so some one please tell me if I am wrong in anyway.... I don't think I am, but it's possible.
