Mat Watson heads to Silverstone circuit to try Audi’s most expensive road car ever, the R8 GT Spyder, for Auto Express magazine. Powering the rag-top supercar is a tweaked version of the regular R8’s 5.2-litre V10 engine, resulting in 412 kW (560 hp) and 540 Nm (398 lb/ft) of torque, allowing the Spyder to rocket to 100 km/h (62 mph) in a mere 3.8 seconds and onto a 317 km/h (197 mph) top speed. So, is it worth the extra coinage?