Hmmm, well I'm pretty sure that if you run stacked boxes, hone the injectors, install twins, 5" off the turbo all the way with no muffler, up rate the fuel system because you will get to zero fuel pressure if you don't, you will in fact exceed the sound volume of a jet taking off when on the dyno as measured with a decibel meter.
Yes I'm evil and did those things. Also since I was leading the charge and among the very first I tested the limit of a stock clutch, lunched a transmission, blew the rail, head gaskets, lots of stuff. That part was less happy...
Anyhow I was down to Rips at one point and we installed a series of mufflers varying in size from small to massive 3 foot long suckers and also straight piped it both in 4 and 5" in each case and took decibel measurements along the way. Racer was right about that at least as it applied to a 3G Cummins, going from stock to straight pipe surely upped the ante but it was not nearly so much as expected. That was on a truck with no cat, no DPF. I'm thinking that as you gents surmised it wont be easy to get much volume on this car unless surgery is performed on the upstream exhaust along with the aid of burning more fuel.
I'm not going to mod this car since I share it with my wife and I'm not so old as I don't want to see her naked again but I'm a performance guy and I'll be curious to see what comes up now that the engine is hitting the trucks which I'm guessing will be a much more fertile market for performance mods.