I'm planning to upgrade my car and was curious whats out there and why the JCW factory car seems to have higher HP? Thanks!

 

http://www.motoringfile.com/2008/11/...ory-jcw-coupe/

The drivetrain received a handful of modifications by BMW (not all by the M division mind you). For starters the intake valves and valve seat rings were upgraded with more durable materials to withstand the increased temperature caused by the free-revving engine. The pistons (and this is where M came into the equation) have had their sidewalls reinforced and subsequently the compression ratio has been reduced to 10.0:1. The cylinder head wall thickness has increased to reduce potential stress during peak output.

The engine breathes better due to the fitment of a larger air mass sensor among other additions meaning greater throughput of cold air and a reduction in the loss of throttle power. Induction is also quickened through a larger air intake. Finally a strengthened and larger twin-scroll turbocharger and exhaust manifold are fitted resulting in a pressure increase from 0.9 to 1.3 bar.
The exhaust and catalytic converter was reworked and enlarged to aid the unrestricted flow and reduction in back pressure. All this means a couple of things. Turbo lag is reduced further (already pretty non-existent) and power and torque are increased to 211 bhp and 206 ft lbs respectively. Yet the Works car is rated at the same MPG as the stock MCS while getting 132hp per litre. It’s so impressive that BMW has already announced that this engine will be showing up in the next generation 1 Series due out in 2012.

 

Here's another article http://theswitchback.com/driven/mini-john-cooper-works ...

 

One additional thing that isn't mentioned, that I can find, in either article is the gearbox. The Getrag 6-speed has stronger internals for the increased horsepower...