There was a LONG debate about this over on the Club3G (Mitsu 3rd generation Eclipse) board a while back. The general consensus was this: NONE of the modifications above will void your 'warranty' so to speak, as your warranty is not an all-encompassing item. What it WILL void are the covered repair / replacement costs of any part you have replaced with an aftermarket part. For example, I have replaced my factory air intake and exhaust on my 3G Eclipse. I have not voided my 'warranty', but I have voided the warranty on my factory air intake an exhaust. Should I have a problem related to either one of these, I'm out of luck. Should I have a problem with my suspension (which I have not yet changed), I would be OK.
As other folks have probably also mentioned, the dealer may attempt to use your installed after-market parts as an excuse not to honor other (unrelated) warranty claims -- your milage may vary.
Hope this helps --
Jeff