I agree with your mechanic and not....
The exhaust gasses are hottest right AT the turbo, therefore they occupy the most volume there as well, the further up the system the larger it should be. BUT, for mild to medium mods, Ford has more than enough pipe for you stock - he's 100% right.
Now, adding size or "low restriction" exhaust down-stream is a joke unless you are building a pulling rig, then it has value. I don't mean that if you EVER pull anything it has value, but if you pull on a track (compete) or pull EVERY DAY for a living, then it has value.
Why and am I on [email protected]
? LoL, no. Exhaust cools RAPIDLY as it leaves the turbo and flows thru the down-pipe, that is unless you are PULLING HARD, then it carries the heat further back. The exhaust and muffler are more than big enough for even mildly modded engines because the exhaust VOLUME has decreased significantly due to it being cooler than at the turbo.
The only way to KNOW if your mods require a less restrictive exhaust is to weld a bung just after the turbo and monitor EBP. If you lock the warm-up device open, you can simply move the EBP sensor to bung on a length of tube to keep it from getting too hot and watch EBP with a scan tool. You will be supprised at how little restriction the factory exhaust has.
Now that I said all of that I need to clarify one thing. That ASSUMES the mods don't increase RPM more than a couple hundred. This is because exhaust restriction is a cube function of flow velocity (RPM if you will), adding RPM (a WRONG but favorite trick of Tooners) to get more HP on the dyno can cause the exhaust system to be a restriction on one truck where another equally powerful truck might have the stock system; due to better programming and not using RPM as a HP crutch.