Heim-joint suspension components
Prepare to shake those fake "M" badges right off the trunk lid. Heim joints work great on racecars for two simple reasons-- there's no consideration for noise, vibration and harshness, all of which are extreme, and they can be replaced every 10,000 miles on a racing budget.
Carbon body parts
Carbon-fiber body parts for street aren't usually the dry carbon weave you'll find on racecars. They're usually wet carbon, and typically consist of a layer or two of weave over a fiberglass body. In addition to not being particularly strong, they're not particularly light. Not to mention you'll be replacing your carefully designed crumple-zone hood with a sliding decapitator. Your door beams designed to protect you from side impacts, but you'll need to remove those to install carbon-fiber doors, and since you shouldn't have a rollcage (see "Rollcage"), you're now a sitting duck. Finally, these parts don't fit well, and they allow enough road noise into the cabin that you won't be able to hear yourself think.
The tiny 2.5-liter engine sitting in the bay of this E90 rips out an astonishing 290+ hp at more than 8300 rpm, but tight tolerances required to create this kind of oomph mean a rebuild every 25 hours at $10,000 each. But the real kicker is the amount of tuning required to make it run right, not to mention the custom wiring and the fully programmable $13,000 MOTEC M800 Pro and MOTEC ADL-2 datalogging system.
That sweet Toora steering wheel definitely adds to the driving experience. Just one thing: It comes at the price of your airbags. The Racetech seats are top-notch for racing, hence the name, but they check in at the cost of comfort, and worse yet, your blind spots, which probably won't pay off on the highway.
Xtrac sequential six-speed. This is as fast as you're going to get if you still want to move your right arm. And if you thought those carbon-fiber doors were making things noisy in the cockpit...
Actually illegal in most states, a side-exit exhaust (located on the opposite side in this case) is a great way to get rid of that unwanted leg hair and exfoliate your skin. If you're a good fabricator you can build a relatively quiet system, but where's the fun in that?
You'd love the added stiffness, but unless you plan on wearing a helmet every day, putting a rollcage in a street car is a fool's gambit. One good knock in traffic could send your skull into a bar and--poof!--you're a goner.
As we all know, downforce plays a critical role in keeping a racecar planted in high-speed corners, and it works great in this application because of the heavy-duty spring rates. Since there's no way you can run racing spring rates on the street, the massive downforce created by a wing like this would serve largely to push the back of your car into the asphalt. But no worries; you won't be cornering fast enough to notice a difference anyway.
Tires make the single biggest difference in the handling department. But while it's perfectly reasonable for the BimmerWorld 325 to polish off three sets of sticky Toyo R888 rubber per weekend (at $800 a set), your street car will need to compromise, like with a set of Michelin Pilot Sport 2s, which have more than twice the treadwear. You won't have quite the same ultimate grip, but street driving is about having fun, not setting lap times. Incidentally, race tires can quickly overwhelm even the best street suspension setup anyway.