If any modified part causes a problem, it will void the warranty on those affected areas; e.g. a CAI won't void the warranty on your stereo, but if it results in killing your engine then no warranty. Subaru particularly had a lot of problems with people changing exhausts, fitting intakes, etc. on WRXs and killing their clutches and gearboxes. No warranty applied because people were putting the parts under more stress than they were factory engineered to take.
Insurance companies also particularly don't like engine management modifications -- even less than many other mods. Many will just flat out refuse to insure you -- because you're more likely to cause damage to the car.
Of course, if you're on a track, you're uninsured for that period of time anyhow. If anything goes wrong on a track, you're not covered...
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