below an extract from rangerovers.net about performance changes Overfinch made to the bmw 4.4 v8 petrol engine

Performance Upgrades
Overfinch boosted the power of the already impressive 4.4 liter BMW-derived V8 in the new Range Rover by increasing the displacement to
5.0 liters and installing a host of other refinements to enhance power, throttle action, economy and emissions performance. A freer-flow dual exit contoured exhaust was added, with polished stainless steel tips. Modified transmission ratios gave a further sizeable gain in everyday performance. The engine was exhaustively tested in punishing environments, including cold weather testing in Russian winters, hot weather testing in the Nevada desert, general urban behaviour in the UK plus high speed work on the autobahns of Germany. The result, known as the 580S, was a boost to 380 BHP, and 376 ft lb of torque. The engine was tuned to maintain high torque values over a very broad rev range, producing effortless performance characteristics. Interestingly the continuing BMW based 5 litre model (580 S) launched in 2003 posted identical 0-60 times in independent tests to those quoted for the new Jaguar Supercharged Range Rover but with less power and without the aid of a 6-speed automatic. Zero to 60 mph comes up in 7.1 seconds, and top speed was about 140 mph. Interestingly, these figures equaled the performance of the subsequent 2006 Supercharged model introduced by Land Rover. Naturally, when the Supercharged version came out, the 580S was discontinued, but you should still be able to find examples on the used market.

Overfinch also made available a 480S Sports Pack for the 2003-2005 models as an add-on package. It included:
- Re-programmed engine management ECU
- Free flow stainless steel exhaust
- Dual contoured polished oval exhaust tails
- New gear ratios
-Overfinch ovals front and rear
- ‘480S’ badge on tailgate

Mid-range torque was substantially improved, and the result is much better response than can be achieved by a simple "rechipping" of the engine. There is a 1 second improvement in 0-60 time and 2.5 seconds improvement to 100 mph.