99EJ6T-
Nope. You've got some of it about the turbo, but you're showing a lack of understanding of exhaust gas flow in an exhaust system. Heck, I don't know as much as I should, so, anyone who thinks I'm coming off like someone who knows everything can go suck a big fat one. I just happen to read enough to get the basics.
There is going to be little to no "tumble" in an exhaust system post turbo, unless the exhaust tubing itself is made very terribly with bad fitments and welding, or it is designed completely wrong. Manipulating volume of the exhaust is critical and key to producing as much power as possible in ANY exhaust system because of the effects on exhaust velocity. Going straight to 3" or 4" right from the outlet of the turbo is just plain silly. Transitions are key in all places where pressure changes occur, especially ones that occur rapidly, like going from low throttle to high throttle.
For example, I actually plan on using a section of 4" piping for my next turbo build. It will transition at a steeper than normal angle from the outlet size to 4", where I'll have about a liner foot or so of piping, which will transition very gradually down to 3" as it turns under the pan, continuing at 3" until after a flex and high-flow spun core cat, where it will neck down very gradually to 2.5" before the center-tunnel mounted muffler, with one 90" to the passenger side, then a turndown in front of the passenger rear tire. I neck it down after the cat, because the cat is the first real restriction, and because of the thermal effects on the exhaust gases after the cat. They'll be cool enough to be reduced in volume post-cat, and slightly decreasing the volume of the tubing will help keep the velocity up. I'm choosing this design because it will provide a very large "transient reservoir" to deal with near instantaneous throttle changes encountered when driving in the real world. Throttle changes lead to wide changes in exhaust gas volumes in very short periods of time. Creating a nozzle shape on the outlet of the turbine with enough volume to accommodate the fast increase in the volume in those transitions will lead to better total engine performance, at least, that is what I am theorizing.
Quite frankly, none of this would really matter in most cases, and for most peoples' builds. Since I am not worrying about maximum power, but throttle response and torque production as low and fast as possible with a larger than usual turbo (for the purpose of throttle response, anyway) with a larger turbine A/R, this is why I have gone through the trouble of learning how manipulation of the volume of exhaust tubing leads to different effects. And, also for the record, I could be completely wrong. But, i don't think I am. *shrugs* We'll find out in a decade or three when I actually have the time and money to do any of this.