My latest infatuation is researching air intakes for the 335i engine. There are quite a few out there now and some really have a great looking setup. Here are few things I've learned looking at intakes:
1. Dual or Single cone? Dual cones definitely give you a higher performance boost and most choices give you about a 12-15 horsepower gain. Single cone benefits are minimal compared to stock so if you are doing this you want some performance gain.
2. Costs range from $130 for a nice BMS dual cone to almost $2,000 for a beautiful, yet insane, all carbon GruppeM model. I find the better choices are in the $300-$400 range for a good looking dual cone setup.
3. Low or high sitting intakes. Some intakes are installed where the filters sit low in the engine bay and this setup can cause water getting easily into the filter's air flow which can lock up your engine. The best choice is to go with an intake that sits high in the engine bay so water is less likely to get into your air flow.
4. Warranty. Replacing an intake could affect your warranty with BMW (or in my case Dinan.) It's a rare thing but intakes affect engine performance and if you have a major engine issue it seems you could get into some sticky discussions on a warranty claim if the reason for an issue isn't crystal clear. So, be aware of this when considering this kind of modification.
There are two dual intakes I'm considering. The best looking one is the Injen model (see image above) that runs in the $400 price range and even comes in a red color with red filters. Sure the red/red thing is a bit "ricey" but there is something very cool about the look that is appealing. The other choice is probably the favorite preference of the E90Post.com board, aFe Dual Filter Intake; it's gain is a supposed 27 horsepower. It's a nice setup in the mid $300 range. It looks good and actually is considerably easier to install than the Injen product.
My big issue with any option after researching all of this is warranty. I have the Dinan software and really wonder if any future claims could be derailed if I do have an engine issue before my 4 years or 50,000 miles are up. For this reason, I may just wait two and a half years and do this modification and look at getting the BMW Performance Steering Wheel now.