what i mean by this is ,does a smaller plenum bump up low-end torque some? or does it help top-end rpm? say if you were to add metal inside the plenum so that it is smaller. i plan to do this on my saturn's manifold. i know it wont do much, but i want to concentrate on low-end, since i don't race it at all.
modifying your plenium chamber to make it smaller will probably cause you overfueling problems.
Making the plenium bigger can assist in race engines and allow a greater volume of air/fuel to be available for the engine at high revs generally this is also complimented with a air scoop.
To obtain the same result that you would get with a smaller plenium you could fit a quieter(more restrictive or extra) muffler because decreasing the exhaust flow will raise the torque a touch. This is why some engines lose their low end when the standard air filter and mufflers are replaced with freer flowing ones; the engines rev higher and horsepower goes up a bit but the torque starts slightly higher up the rev range.
I had a 283 chev in a toyota hilux(forerunner) 4WD and put freeflowing everything on it and lost alot of low end torque so i put more restrictive mufflers back on it and it improved the low end torque.