Some items for performance:
- Cold air intake - This will replace the OEM airbox and provide colder air for more power. You should see 5-10HP gain. I personally prefer Needswings (Performance auto parts & aftermarket auto accessories for your car, specializing SRT4, Crossfire SRT6, SLK32, C32 AMG, VR-4) configurations. Rob (owner) is a great guy to do business with.

- ECU tune - There are only a handful of respected companies that tune our cars properly. I personally prefer Eurocharged (www.eurocharged.com). They have treated me quite well and I have been impressed the results. You should see 20-30HP gain with a tune. Eurocharged can remotely tune your car over the internet as well.

I'm sure that others will chime in

For wind defusers, there are a couple of different companies out there. When I had my convertible, I used the King Penn Windrestrictor (www.windrestrictor.com). Their products works well and can be custom made with etchings and lighting. Others have used a full fabric one that goes across the back of both seats. Do a search on the forum and you should see plenty of information on this topic.