SUBARU'S iconic Impreza WRX STi can now be fitted with a 305 PS performance pack, giving a 155 mph top speed and 0-60 mph time of only 4.6 seconds.

Developed by Prodrive, the power boost costs £1,995 including 2.5 hours for dealer fitment.

Initial supplies begin this month, following extensive durability tests.

Not only does power leap from 265 PS (261.5 bhp) to 305 PS (300 bhp) at the same 6,000 rpm, but torque rises from the standard STi's 253 lb ft to 299 lb ft, also at the same 4,000 rpm.

The result is not only startling high-rev performance but also more low and midspeed flexibility. In other words, the car becomes swifter at any speed and in any gear.

Side-by-side tests revealed a 0-60 mph time down from 5.2 seconds to 4.6 seconds and a 0-100 mph figure of 12.2 seconds instead of 13.7 seconds. Quarter mile figures are now 104.7 mph/13.4 seconds instead of 101.4 mph/14 seconds.

The 30-50 mph time in fourth gear drops from a still impressive 5.6 seconds to 4.4 seconds while the 50-70 mph time in fifth is now 4.9 seconds instead of 6.1 seconds. Further up the speed scale, the 80-100 mph time in sixth gear is covered in 6.5 seconds rather than 7.8 seconds.

The performance pack produces more pulling power between 3,000 rpm and 5,800 rpm than the standard STi has at its peak.

The pack - which carries Subaru’s full three year/60,000 mile warranty - comprises a revised engine control unit, high-flow catalyst and sports silencer.

Since launch in January 2002, the Type-UK WRX STi and STi Prodrive Style have been a tremendous success, accounting for more than 30 per cent of Impreza sales. Al1 750 Prodrive Style models are sold and order books are still strong for the standard STi which costs £25,995 on the road.

Key features of the WRX STi Type-UK are inverted suspension struts, Brembo brakes, a new six-speed gearbox, front and rear Suretrak limited-slip differentials, a faster steering rack, larger wheels and tyres and a standard satellite anti-theft tracking system.

Meanwhile, the standard Impreza WRX continues to be available with the popular Prodrive WR Sport performance pack which costs £1,600 including fitting.