SO MUCH more than just a Boxster with a roof, the Cayman S poses a serious challenge to the much more expensive 911 Carrera as the Porsche of choice for the committed driver on a (relatively) tight budget.

The Cayman shares its mid-engine platform with the second generation Boxster convertible - including the doors, headlights, taillights and much of its interior - but is marginally more powerful, faster and more enjoyable to drive than its soft-top sibling.

Porsche justifies charging more for a tin-top (contrary to every other car manufacturer) by saying the Cayman represents the flagship of its two-seater range. It also maintains the Cayman shares more components with the substantially-more expensive 911 (51 per cent) than it does with the Boxster (29 per cent).

There are some visual differences. The Cayman's nose is more aggressive than the Boxster, with a wider central air scoop beneath the number plate and different LED signal lights.

The direct fuel injection 3.4-litre engine that sits behind the driver is the same as the one in the Boxster. It’s good for 320bhp, a 10bhp advantage, but, once you’re on the move, it feels like more.

The flat six produces the same guttural grumble as a 911. As it's mid-mounted you can hear all the induction, compression and ignition going on just behind your head. It doesn't send a shiver up your spine like a Ferrari, or even a GT-R, but there's a mean no-nonsense note to the exhaust that hints at good things to come.

The PDK seven-speed semi-auto gearbox is a great option. It changes gear faster than you could move your hand, increases performance and reduces fuel consumption and CO2 output - what’s not to like?

Fitted with PDK and launch control (featured in the optional Sports Chrono Package Plus) the Cayman S accelerates from 0-62mph in an impressive 4.9 seconds. Coincidentally, that’s exactly the same time as the 911 Carrera.

The Cayman’s snappier throttle response is thanks to the new direct fuel injection system. Fuel is squirted into the engine cylinders under very high pressure milli-seconds ahead of combustion, leading to a better fuel/air mix and more complete combustion. That’s one reason (the PDK transmission is the other) why the Cayman is 15 per cent more fuel efficient than it used to be.

The Boxster is no shabby handler, but the Cayman - with a metal roof in place of a floppy soft-top - is better.

Unleashed, it doesn't drive through tight and twisty B-roads it dives in and dismantles them. The chassis corners flat and true, it changes direction in an instant and the tyres grip like there's no tomorrow. If you’re planning a few track days, the optional limited slip differential gives even more rear end grip and is £700 well spent.

Best of all, you can enjoy the Cayman's telepathic handling ability at perfectly legal speeds by dint of it's ability to slingshot around bends at least 10mph faster than you previously thought possible.

Press the sport button and the Cayman is quicker still and there’s the sport Sure, the 911 has a higher top speed but cars like these are so quick that maximum mph figures are academic. Does it really matter if your Porsche does 180mph or "just" 177mph in a country with a legal limit of 70mph? No, it doesn't.

You don't have to take my word for it. Former world rally champion Walter Rohl, who helped develop the Cayman, reckons that, while a 911 may still be the track hero, a well-driven Cayman is faster in the real world.

Shortly after the Cayman's launch, he admitted: "On a very twisty road I believe, yes (the Cayman is quicker than the 911). In second and third gear corners it is more precise - it is so, so competent."

Walter's not alone in wondering at just how close to the 911 the Cayman turned out. After his first go, Le Mans 24 Hour winning racer Andy Wallace described the Cayman as the "best road car I've ever driven".

And don't forget that a privately entered Cayman finished fourth overall in the 2007 Nurburgring 24 hour race, ahead of two Porsche flagship GT3 RSRs and a phalanx of supposedly faster cars.

Maybe it’s wrong to think of the Cayman as a more expensive Boxster. In terms of how it goes and drives, it’s snapping at the heels of the 911 - and a Cayman S costs a cool £18,000 less.

The Cayman is every bit as well built and usable (the rear hatch holds 260-litres and you can squeeze in another 150 litres beneath its nose) as the iconic 911. You get the same opportunity to visit the factory and collect your car off the line and the same offer of free performance driving lessons at Silverstone.

I guess the question has to be: just how much value do you place on the 911’s two rear seats?

PORSCHE CAYMAN S.

Price: £47,411 (PDK.

SPEC CHECK: Engine: water-cooled six-cylinder aluminium boxer engine.

Max power: 320 bhp @ 7,200 rpm.

Max torque: 273 lb/ft @ 4,750 rpm.

Top speed: 172 mph.

0-62mph: 4.9 seconds (PDK with launch control).

Fuel consumption: 26.6 mpg on test.

CO2: 221 g/km.

Equipment: Electric windows, alloy wheels, leather interior, CD player (Bose option), air conditioning, electric mirrors, driver and passenger airbags.

SPEC CHECK: Engine: water-cooled six-cylinder aluminium boxer engine Max power: 320 bhp @ 7,200 rpm Max torque: 273 lb/ft @ 4,750 rpm Top speed: 172 mph 0-62mph: 4.9 seconds (PDK with launch control) Fuel consumption: 26.6 mpg on test CO2: 221 g/km Equipment: Electric windows, alloy wheels, leather interior, CD player (Bose option), air conditioning, electric mirrors, driver and passenger airbags.