When it comes to monsters lurking under the bed, few are more ferocious than the Audi C7 RS6 Avant.

There’s more than a hint of its fearsome ability in its plump arches, 21-inch dished rims and low-key stance, but nothing can really prepare you for the absolute mind-altering sensory onslaught that a hit of in-gear acceleration burns into your very soul. It is so freaking fast.

Being a $230,000 Audi, it’s also pretty well equipped out of the box, with niceties like LED matrix headlights and a wickedly comfortable and good looking interior. And it’s this level of savvy sophistication and speed that attracted BWA Auto’s Bob Whyms to the RS6 –that, and the fact that it was a screamingly good deal for such a quick car.

BWA’s staple is Volkswagen AG product, and Bob admits that the company sells far more APR tunes for cars like the VW Golf R. “We recently got a Golf R down to a 11.87 (400m) just with a tune,” he says.

BWA has had the car for about 11 months, but the list of mods is actually reasonably short. There’s the signature wrap, of course. There’s a stage two ECU tune and turbo downpipes from APR, custom MRC turbo inlet pipes and a BMC air filter. And that’s it.

Bob agrees it’s harder to find easy gains with aftermarket bits these days, but forced induction still offers some wriggle room. “If it’s a naturally aspirated Porsche, forget it. They’ve tuned it pretty well,” he says. “With turbocharged cars, though, it’s a different kettle of fish.”

The RS6 is built tough, and the stage two tune has been thoroughly tested both here and overseas – but as with all modified cars, it’s about balance.

The costs of the upgrades totals a hair under $11,000, with the ECU tune costing $4320, the downpipe set $3500, MRC carbon turbo inlet set at $1200, the BMC air filter set $240 and labour coming to $847.

“If you look at the car, it’s $200K plus on-roads as a brand-new car, and for 5 per cent of the value of the car, you can get an extra 25 per cent performance improvement out of it in terms of power and torque,” says Bob’s son Craig Whyms, who also works at BWA Auto. “That’s the sweet spot for the car. Whereas if you then take that from 10 grand to 40 grand, it’s such a huge jump to get to that next stage of performance.”

“It’s pretty quick,” says Bob. “We’ve gotten an 11.05sec (400m) out of it with pretty worn tyres. It’s capable of mid 10s.”

At an off-street drags event at Western Sydney International Dragway, it even blasted to 201.59km/h by the end of the quarter mile. Given MOTOR recorded 3.93sec to 100km/h and 11.89sec over 400m (192.75km/h) in the equivalent standard RS6, that’s a meaningful jump in pace.

“All our tunes are from APR, which is probably one of the biggest Audi/Volkswagen tuners on the planet,” he says. “Before they release a calibration and a tune for a new vehicle, they put it through the paces and try to find the breaking points, and then come back from there to make sure that everything is still within the safety limits and within the ceiling of what the car is capable of.”

I’ve sampled the carnal delights of the RS6 before, and it’s an astonishing thing in stock form, with loads more power than is necessary in a four-door wagon. And this one has another 100kW and 300Nm? Gulp.

“There’s a good straight bit here,” says Bob. “Give her a go.” And I do. And oh my freaking lord… a tumultuous, relentless tornado of torque wells up as you depress the throttle, accompanied by that distinctively Audi V8 staccato bark that’s been turned up to 11. The RS6 simply rockets forward, with corners coming up almost as fast as you can blink. Christ on a cracker.

On light throttle, though, it’s completely normal. The twin-turbo motor doesn’t rev as high as, say, Audi’s old 4.2-litre atmo V8, but boy, does it hit its 5700rpm redline quickly.

“I know APR had a developmental stage three car and even at that point, and they were talking getting close to 1000 horsepower, the transmission was just eating it up,” says Craig. “That ZF eight-speed gearbox just amazing. It’s an amazing bit of kit.”

Punctuating the whole show is an exhaust soundtrack that should be bottled as a tonic… and it’s largely stock! Inside the car it’s just as giggle-inducing as it is in tower-buzzing mode, too, with more pops and crackles than a Kelloggs factory explosion punctuating every throttle lift and every cog swap.

The BWA Auto crew have enhanced one of this generation’s most potent performance sleepers with a balanced tune that adds to the excitement without spoiling the RS6’s inherent balance and civility. It’ll tear your head off if you want it to, but you can also tow a two-tonne boat with it if you needed. Try doing that with a 911 Turbo.

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Just Looking Early RS6s are getting (relatively) cheap

Released in late 2013, the RS6 C7 retailed for around $230,000, before being superseded by the upgraded RS6 Performance in 2016, which added $16,000 to the ticket (as well as more power and torque). Factor in on-roads, and Whyms’ 2013 car would have cost more than $300,000.

“I paid around $170,000,” reckons Whyms, “with around 11,000km on it.” Add $10K of mods, and Whyms’ RS6 now has more grunt than a $760,000 Lamborghini Aventador. Perusing the classifieds, there’s even cheaper RS6s around, with a couple for $140,000.

LIKES: Makes a bigger mountain out of a mountain DISLIKES: Further work will cost serious bucks

RATING: 4.5 out of 5 stars