An intercooler does help drop the charge temp (it's main function). Whether it's a 20-40 degree difference before entering the turbo or not via CAI, the intake charge is still getting heated up substantially, hence why a good intercooler is better than a CAI imho.
Most of the cold air intakes I've seen sold for these cars are really, by definition, short ram intakes. Although the proper use of a heat shield goes a long way into making use of the cold air coming from that side of the bay as opposed to the hot air of the rest of the engine bay.
And yes, a CAI can arguably void your warranty depending on what caused the issue in the first place and if the problem can be traced to the modified system/part. If I remember right, the Magnuson-Moss Act covers aftermarket parts on a vehicle causing the warranty to be voided, unless your warranty specifically states ANY modified pieces will cause a void and you signed the contract anyways. Then you get into a legal grey area. The fine print is the final say essentially, unless of course the dealer can prove without a doubt that the part caused the issue being repaired under warranty.