In this universe, the Boxster isn't a roadster of neutral gender, it's as butch as Bruce Willis in a sweat-smudged vest. It has a thick, chiselled jawline and gaping, 911 Turboesque air ducts. No longer does the front look like the back. And those arches are positively screaming for air: each 19-inch forged magnesium wheel with its fat Pirelli P Zero Asimmetrico cover fills the space like a knuckle in a socket. Don't panic. It's more than a Boxster in a body kit. At idle, the exhaust note is throbbier, throatier. On the move it's louder and lairier. In fact, this car sounds and, in a straight line, goes exactly like a 911.

Which is only surprising if you believe, as Porsche does, that the Boxster is a sidekick to the 911 with power and performance deliberately suppressed to flatter the more famous model. Bavarian Porsche modifier Ruf doesn't think that way. It believes in natural selection: so why shouldn't the Porsche with the best chassis also get the best engine? Anything less seems rather perverse. Think of it as that most delicious of 'what if' daydreams: a mid-engined 911.

Ruf is daring to sell a car that Porsche has probably made but, for fear of commercial suicide, kept to itself: a ballistic Boxster, the best of both worlds.

It's basically an engine transplant. Out goes the Boxster's regular flat-six power plant - which, even in 3.2-litre 'S' tune, delivers a comparatively modest 252bhp - replaced by the slightly bulkier 3.4-litre watercooled flat-six mill from the previous 911 Carrera, sitting on lowered engine mounts so that it fits and preserves the Boxster's centre of gravity. The Boxster's six-speed 'box remains, but now sits in front of the engine rather than behind. There's 50 extra bhp on the plate, not to mention another 40lb ft of torque, which has serious implications for the Boxster's ability to propel itself down the road. A second is sliced from the 0-62mph time (5sec) and 14mph added to the top speed (175mph).

In addition to the high-tech wheels and rubber (235/35 ZR19 and 275/30 ZR19), the suspension uses uprated Ruf springs and dampers and the ride height is dropped by 20mm at the front, 10mm rear. The generously-specced Boxster S brakes, though, are more than capable of retarding the extra thrust.

Ruf's 3400S really does inhabit another dimension - visually, dynamically and emotionally. Inside, the incongruously cheapo click of the column stalks still disappoints but swapping the Boxster's instrument pack for the 911's (two more dials) and replacing the perfectly adequate standard seats with the GT3's superb hip-huggers massages the fiercely functional ambience. What matters, being a Porsche (or a Ruf, for that matter), is that a big rev -counter lines up in the centre of the steering wheel, the seats locate and the driving position is perfect. Few cars are as fearlessly driver-centric as the Boxster.

But twisting the Ruf's chunky key in the ignition immediately reminds you how different and special the 3400S sounds and feels. Its 3.4-litre motor is an altogether stronger proposition: sweet spinning and smooth, yes, but distractingly more muscular over a broader rev band. The sustained push is pure Carrera but deliciously contained in an even more compact and pointable car. For the first time in a Boxster, it feels as if your right foot is hard-wired to some heavy-duty stonk. It is. Keep the engine above 3000rpm and any 911 you come across this side of a GT3 is in for a shock.

Throw in some swerves and the Ruf's case grows stronger still. The firmer springs and stickier rubber don't exact too severe a penalty on ride comfort but they dial up truly stupendous grip, traction and stability on smooth tarmac. The downside is accentuated edginess if you kill the power mid-bend, but body motions are brilliantly controlled and the meaty, direct steering is an open window on the road surface. The communication and composure inspires levels of confidence not experienced even in the latest rear-drive Carrera.

Of course ΂£55,000 is steep for a Boxster. But if you regard the Ruf 3400S merely as a Boxster with more poke, you haven't fully appreciated the roll of the dice. The Boxster's a real life hero; the Ruf shows us a more perfect world.