Yeah if you replace the downpipe you
need a tune.
You may have to play with the gaskets and see what works best. The problem with the OEM downpipe and the 3in exhaust is that will have a small pipe leading into the bigger midpipe (and I can't remember if the bolts will line up). In the pic below the OEM downpipe is on the right. The donut gasket may provide a better seal but if it sits too loosely the 3in gasket would be better.