Yeah I think there would be a lot of lag. For one as the exhaust goes to exit the vehicle it does cool slightly which would lower pressure getting to the turbo causing it to take longer to spool, not to mention all the extra piping. most STS setups you remove the muffler, turbo will quite the exhaust on its own. If you did do this I would wrap most of the exhaust in heat wrap to keep it hotter. Also keep the stock manifold as it will keep heat in better.