I originally pulled 406 RWHP with just a CAI and catback. There is a lot to gain with just letting these cars breath. Much more than that and you risk loosing warranty. However, you may want to check around, there are some dealers who are willing to do warranty work on about anything from what I've read. Lots of people going to the handheld programmers, like predator or superchips, instead of the installed chips so that the can flash the PCM back to stock and leave no visible signs.
In the end, if you are going to tinker, be probably should be prepared to pay.