Of course theres a restriction, theres no way a 3.5 inch straight through will be quiet without one. I think it would be a great setup if you could hook it up to the ecu for load actuation, or apparently there is an addon Xforce sell that automatically actuates the valve on engine revs also. (similar to what Ferrari etc use)
My question to you is, can you guarentee a customer the Hooker system is quiet enough to not get dicked? They are built for the "note" V8 guys want, just like Magnaflow mufflers and unfortunately that makes them too loud in most cases.