A german magazine (Auto-Bild) produced a comparative test between the SRT-10, the Gallardo and the SL55-AMG. The Gallardo came out first and some figures/results are interesting to reproduce here
SRT-10: 8.3 litre, 711NM/4250 torque, 506 BHP/5600, 0-100 4.8 secs, top speed 306 kmph, test consumption 20.8 litres per 100 km., test weight 1585 kg's. Price 105000 euro
Gallardo: 5.0 litre, 510NM/4500, 500 BHP/7800, 0-100 4.5 secs, top speed 309 kmph, test consumption 18.7 litres per 100 km, test weight 1640 kg's, price 148480 euro.
SL: 5.5 litre, 700 NM/2750, 500 BHP/6100 0-100 4.7 secs(autobox), top speed 250 kph, (governed), test consumption 17.7 litres per 100 km, test weight 2010 kg's;price 126556 euro.
So what can we conclude: The Viper with the biggest engine and the lowest weight cannot keep up with the Gallardo, and not even with the Benz, although its 400 kg's heavier. It will take the most fuel of all three, but can only take 70 litre, forcing you to refuel every 300 km or so. The Benz recorded fuel consumption is probably less because it was not tested to the topspeeds of the other two because it is governed.
Although the price of the Viper is 50% lower than the Gallardo, and 25% lower than the Benz, the German magazine observed that it's behaviour is so tricky especially under wintery or wet conditions (there is no ESP) that you would better be advised to use it between april and october. (It would save you some road tax too).