Choose your noise level. Bigger is louder, all else being equal.
The "5 inch straight pipe is so big it doesn't have enough backpressure to spool the turbo" statement is hogwash. Differential pressure is what causes the turbine to spool, and higher backpressure after the turbine decreases that differential. The IDEAL turboDiesel exhaust is a short, straight pipe right off the turbine. Any more than that is gas friction, and friction requires (uses) energy/power. Of course, we don't do this, or we'd have the exhaust dumping into the cab. The actual length, size and shape of the pipe can be tuned to create and optimize laminar flow, which develops the least friction (this isn't usually desirable, because the exhaust noise is slowed less, too). A pipe that's too large, too soon, can reduce power, but only because the gases cool quicker, and cooler gas is more dense, more turbulent, and this equates into friction. All the backpressure for an effective turbo happens between the cylinders and turbine. After that, it's just waste. A N/A engine (Diesel or gas) is different, as they are able to scavenge exhaust, which can increase power, and headers can help optimize this advantage.