With a power output of 621 bhp and 1000 Nm (737 lb ft) of torque, you could be forgiven for thinking these are the most significant numbers attached to Mercedes’ new V12-engined SL65 roadster, but owners will quickly discover that the car’s depreciation is far more brutal.

You see, with the 529 bhp/800 Nm SL 63 AMG costing £106,820.00 and the 563 bhp/650 Nm SLS AMG Roadster requiring a bonus-quenching £176,895.00, you’d need a seriously good reason to spend £162,000.00 (est.) on the V12 SL 65 AMG (the 2009 model retailed at £153,050.00).

Most customers will chose the significantly cheaper SL 63 AMG, which is more than fast enough for even the straightest of autobahn, and if ‘flashing some cash’ is your raison d’être, then look no further than the gorgeous SLS AMG, which brings the added bonus of being the official Formula One Safety Car.

As with the SL 63 AMG, the new SL 65 AMG is some 170 kg lighter than before, thanks to its all-aluminium bodyshell, there’s also a new AMG Speedshift Plus 7G-Tronic transmission replacing the old 5G-Tronic system, so power is delivered even more smoothly than before.

But the SL 65’s real party-trick is that engine and its ability to catapult you towards the horizon with no discernible limit, except the courage to keep your right foot pinned to the floor. The old model could reach 100 mph in around 7.5 seconds and cover the quarter-mile in 11.6 seconds (Car & Driver figures), whilst this 2013 model is quoted by Mercedes as being capable of 62 mph in 4.0 seconds and 125 mph in just 11.8 seconds.

The AMG 12-cylinder bi-turbo engine impresses more for its effortless performance, than sporty feel, but for a small and loyal community of fans, that’s precisely why they choose to buy one.

At 1950kg, the SL 65 could never be described as ‘lightweight’ (despite the meaning behind those initials) but will still average 24.4 mpg (combined) with CO2 emissions of 270 g/km. The 105 kg lighter SL 63, by comparison, achieves 28.5 mpg (combined) and 231 g/km of CO2 emissions.

It’s hard to argue against the 35% less-expensive SL 63 AMG, but if you must have your roadster with all the toppings, then the new SL 65 AMG will be a riot of fun to own.

Mercedes are yet to confirm the SL 65’s final UK pricing and specifications, but it’s available to order now and will arrive in the UK from Autumn 2012.