the blow off valve is more importent then the intercooler, but you really do need both. The bov is gonna vent the extra pressure that is still in the chargepipes when you shift, this prevents it from rushing back into the turbo and damaging it overtime. The intercooler makes it possible to run more boost safly, and make more power from the boost your making. the turbo is spitting out hot air, so simply putting a charge pipe from the compressor to the throttle body isnt gonna lead to very impressive numbers. Now installing an intercooler, allows for that hot air to first get a cooled, which will dramticly reduce intake air temps and allow for more power and less concern for knock