I had the same feeling when I thought of buying the Stinger GT1...
Give it time, though, and the soundproofing will be fixed. It was the exact same complaint Mazda folks had for the first 4 years of the CX line, and when Mazda too decided to make a run to tackle the luxury brand, the first thing they did was take on the decibel levels in the cabin. And now, it actually is quieter than the Lexus that it most closely competes with, imagine that...
They will also cough up a couple more ponies out of the powerplant to offset the couple of hundred pounds of sound deadening material used to quiet the cabin. Another win win...
The fake sound out of the speakers really bothered me as well, it just seems so teenage ricer, however, it can be turned off, and I'd tackle the problem by creating my own sweet sounding exhaust anyways...
With it all said, I'd still buy it over the BMW counterpart, but I think I'd buy it used with 10-15k miles on it because the warranty transfers, and a 100k mile powertrain warranty is hard to beat, and they depreciate still way faster than other brands, in fact, I would've splurged on one a couple of months ago at a ridiculous price ($32k) if it wouldn't have been white...
Overall, both Mazda and Kia are doing a mighty fine job on their push for the upscale market, I still prefer the Mazda, hands down, I just wish they'd come out with something with the power of the Stinger GT in both a sedan and CUV/SUV, then they'd be really hard to pass up for anyone, given the time to compare and give it a shot, I for one, would buy one today...