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I have restrictor plate and reduced venturi size experience, but... in both instances, they were race applications.
I can offer a nugget of insight as to the effect on peak rpm and torque.
Again, one particular normally aspirated drag race engine with huge intake ports I had on a friend's dyno, a 50mm intake valve, and no regard to fuel consumption (2hp/cu in, if you're interested);
When we reduced the venturi diameter from 50mm to 48mm, peak torque was acheived at 300rpm lower with no reduction in torque at 8000rpm. Reduced again to 45mm, peak torque was acheived at about 100rpm lower, but the torque fell off more sharply at higher rpm, having a peak torque band narrowing effect. Throttle response was 'snappier' with the smaller venturis, not something you normally measure.
I'm with Daox, optimize the port runner diameter and length, thottle body diameter(s) for peak torque where you want it. The other thing to note is that when you start changing the place in which peak torque occurs, or alter the shape of the torque curve itself, you realy need to build a tranmission around the new combination in order to take advantage of the new charactaristics.
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