Im pretty sure it won't completely void your warranty. But if something was broken due to you lowering your car, then yeah, it won't be covered under warranty. If you're really worried about your warranty, you can always lower it will the aspec suspension, and have the dealer install it. It might be really expensive, but at least you'll still have your warranty.
don't worry about it, I was like you worried about installing aftermarket parts while still in warranty but unless they can prove that the fault was due to the suspension (or whatever aftermarket part you have), they will cover it under warranty.