The stock ISV isn't designed to control boost, it can be modified to do so, often on the corrado the ISV leaks, and you do loose boost, this is a very common problem, and you need to do an ISV rerout, which basically means add a check valve in line with the ISV. This would not be the ideal route for a BOV. On the bottom of your throttle body (the piece that bolts onto the intake, and your throttle cable connect to to control the throttle plate) the bottom has a wastegate, which will have a return line going back to the supercharger, if you remove the return line, make a blank plate or add a second intake to your supercharger, you then have an open hole.. You can add a BOV here, inbetween shifts your wastegate opens, and all your boost is lost, not a real big deal since the charger is still spinning, and will have boost as soon as you go WOT. But... if you have long intake lines, and a large intercooler, and it is just a shift it is nice to keep some boost, when u are not on the throttle you can not have full boost, your car will run lean, and detonitation will accour. You can set your BOV at say 5psi, and this will give you a quicker return between shifts, if you are running at like 20psi for instance, and you drop to zero between shifts, it will take longer to gain to full boost under a load, then if you are starting at 5psi instead of 0.
Now on the other hand installing a filter instead of a BOV or the return line is the most efficent thing for your engine. Your supercharger is plenty capable of pulling in enough fresh air, that their is no need to recycle air, and your charger makes air hotter, so why would you want to feed in air you already heated up, when their is fresh air available, this helps keep air temps down which creates denser air, and therefore more horsepower. which I think is the main goal here..
hope this sums it up...
