Different states have different laws. To find out if ground effects are legal in your state, you will need to contact your local department of motor vehicles. You will need to do the same to find out the legal horsepower for a 'street legal' car. If you are planning on moving or vacationing, contact the department of motor vehicles for the states you plan to drive in. Please keep in mind that any customization to your vehicle may decrease your chances of finding an insurance company that will cover your vehicle (even for non comprehensive and collision coverages).
If you have a hard time trying to insure your vehicle, you can click here to get a quote from auto insurance specialists.