While much of what Johny has said is quite true, I'm going to disagree with his comment about the benifit if a dashpot type carb on a drawthrough setup. There are doubtlessly some benifits to be had on a drawthrough turbo set up, (I.E. keeping the off-boost manifold vacuum high to maintain a strong signal to the venturi, and sustain crisp trottle response) but these benifits aren't so critical on a supercharged engine. When you open the throttle on a blown engine, the blower instantly begins pulling air. And can reach maximum boost in the time it takes for the pedal to reach the floor. This gives superb throttle response, and means that greater gains can be acheived by having as little restriction to the blower intake as possible when the throttle blades of the carb are at WTO. On a N/A motor, there will ussually be very little or no manifold vacuum at WTO. But the space between the blower and the throttle blades on a supercharged engine can register enough vacuum to prevent the powervalve from opening. (On a holley) Hence this is why externally referencing the powervalve is a standard modifiaction to a holley when fitting to a blower. (I did that to mine)
Also, another pitfall with the holley's on an in-line engine is accelerator pump discharge nozzle dribble. The holley's use a small needle as a checkvalve on their accelerator pump. On an inline engine the motion of the engine rocking, (Especially with a lumpy cam) causes this small needle (which is only held in place by it's own weight, 1-2 grams) to unseat. This leads to the airspeed through the throats of the carbs, to syphon fuel into the barrles though the accelerator pump discharge nozzles. This problem even exists when the choke housing has been milled off. I emailed Holley to ask for advise, and they put my onot a bloke named Scotty Mclendon, of Mclendon Carburator. he sold me a kit consisting of a new check needle, and a spring including a few shims to set the amount of preload on the needle. Since the accelerator pump actuator is spring loaded, if the sping presssure on the check needle is too great, the spring on the actuator arm can't over come it, and a hole in the fuel delivery results. To be 100% honest, I still have a whisker too much prelaod on my check needle, and there is sometimes a flat spot. (if you look at my dyno sheet in my album, you'll see the dip in the curve right at the start.)