You can't get much more stealth than this.
You could argue that building sleepers is one of the more difficult aspects of tuner culture. Not only do you need to build a car that'll roll on anything that looks crazier than it, but you need to do so under serious restrictions. That includes exhaust note, brake size, wheels, aerodynamics, and a thousand other things. So, when someone out of the UK built a sleeper with power figures to rival a Veyron, we had to cover it.
The car in question is Hass (not F1 Haas) Performance's Audi S8. It makes a hilarious 1,000 hp. Or, put in other terms, it makes an entire Volkswagen GTI-worth of power more than a Lamborghini Aventador Ultimae.
The car is said to be the only single-turbo Audi 4.0-liter V8 out there. It's certainly the only one in the United Kingdom, but it may also be the only one on the planet. After seeing power figures like that, we can't imagine it stays that way for long. Especially because this engine is also shared with the RS6 Avant and RS7.
Regardless, the Garrett G4200 turbo now sits smack in the middle of the V8's banks, just like the orignal two turbos did. Apparently, that was about the only place to put it. New piping had to be used to reroute all the necessary gasses to the new turbo, and a new wastegate was necessary to account for the surely massive spike in boost pressure.
The engine also needs a number of fueling upgrades like a high-pressure fuel pump to make that 1,000 hp. Additionally, the water-methanol system feeds 3,000 ccs of meth into the engine in this configuration.
Aside from the utter insanity under the hood, you'd be hard-pressed to pick this car out of a crowd. A set of new (possibly Niche) wheels and a slightly revised diffuser are the only giveaways. Even then, those say "my car is modified" not "my car will almost certainly kill me one day."
Inside, things are much the same, with a new and well-integrated readout handing off more data to the driver. There's also some odd gauges and the like, in addition to a new fogred carbon wheel. That aside, you wouldn't know what this was until you'd already lost a race to it.