By the letter of the warranty, you can only use the SPT intake/heatsheild and the SPT catback for power mods.
Any engine management, reflash or piggyback, any downpipe, an STI tmic, a different manifold (including an STI manifold), all of that voides all coverage of the engine, transmission, rear diff and axles.
The magg-moss act is irrelevant as any of those mods are designed to change the factory designed and intended power output, and the operating parameters of the motor. This means that almost any failure, in any of those systems, can very easily be attributed to the mods. And its incredibly easy to convince a judge (or in 99% of cases, the court arbitrator) that the mods had something to do with it.
Yes STI suspension parts void the warranty, if you are speaking of parts directly off an STI. The struts and springs in particular definately will due to the big ride height changes. At that point wheel bearings, all noises squeeks and rattles, the steering rack and tire wear can all be denied coverage.
Your insurance company could care less if you have 700whp. They wont not cover you. However, do not expect that they will cover the cost of the aftermarket parts in the event that your car is totalled. For this you must get a "stated value" policy, where you show documentation on the parts, and your vehicle is appraised, then insured for the correct value. This is expensive, but if you have 50k(including the price of the car) into your subaru (which isnt tough to do), and it gets totalled, its well worth it.
Bottom line, if you are concerned with warranty coverage, limit your mods to what is in the SPT/STI parts catalog that they have at the parts counter, and to parts specifically designed for the correct year and model you drive.
HOWEVER, some dealerships are willing to turn a blind eye to some mods. Some will cover about anything. Others follow the warranty policy to the letter. Dealerships that cover cars with mods that should have been denied per the policy, are taking a BIG chance, as if SOA catches it, they have to eat the cost of the repairs, AND they face big time scrutinization from the warranty people, which is a big PITA, and ends up getting legit warranty claims denied in some cases.
<< Worked as a subaru dealership tech, and as a subaru sales person.