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 or like agency. They should be able to tell you what equipment is legal in your state or guide you to the right state agency that can give you this information.
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 DMVs for the states you plan to drive in as well to make sure what is legal in your state of registration would be legal in other states that you would drive through in your altered vehicle.
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 without physical damage coverages of Comprehensive and Collision). Speak to your agent about any customizations you plan on doing to your vehicle to make sure your car would still be covered. For some custom items you may need to add an endorsement for them to be covered by your auto insurance.
If you have a hard time trying to insure your vehicle, you can click here to get a quote from auto insurance specialists.