Please see the performance forum, but as long as we're here I'll post an answer.
A pulley or pulley kit (The kit contains extra items and varies by tuner) costs from $350 to over $2000. Hp gains are 20 to 30 or more at the rear wheels.
There are chips which claim to the same gain, but don't seem to deliver, at least over a wide power band. Chips start at about $500 and go up. Many people are using both, but you need to make sure the chip is optimized to use the increased boost.
A potential source for big hp gains is a more efficient intercooler. Currently, aftermarket intercoolers for MB's are very expensive, but some cheaper ones are in development.
The stock intake is very good. One person has spent hundreds on developing a custom intake and on modifying a Pipercross intake. The increase in hp was less than the accuracy of the dyno.
The exhaust is the same. You can spend big dollars for mufflers and car back systems with little gain in hp. Change the exhaust only if you want a different sound. There are no headers available at this time.
If you really want to spend money, Renntech can fix you up with all of the above plus new cams, increased displacement, and other goodies for almost $19000. You'll have 272 hp.
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