Sorry to hear of your misfortune, but the good thing is you caught it, and are documenting it with LS dealer and the manufacturer. I'd start doing oil analysis from this point on to see what your baseline is, and going forward you should be able to track any unusual wear. Your 5 year warranty should cover you too, but as stated already, it seems it will be on you to carefully track any abnormal results that may show up early on oil analysis. This covers you in case there is some dispute over possible premature bearing or other wear/failure. You may be lucky and not have any issues, but in case you do you want ammo to use as needed.
Was the plastic piece jammed in the inlet to the engine or merely held in place when the idiot who installed the filter missed it?
Did oil pour out of the original filter when you removed it from the engine?
Did you by any chance keep the original filter? Or original oil?
I'd also read the entire warranty regarding powertrain coverage and make sure no loopholes exist. FWIW, all garagekeepers have garagekeepers insurance, so if something does go belly up, their insurance would cover it if nothing else did.
One other thought is to try to find out whether the original filter is installed at the factory, or if it's done at the selling dealer. I suspect it's done at the factory, and filled with oil there. If so, what exactly is used prior to installing the filter, to protect the filter's mounting threads on the filter nipple? If it turns out to be a plastic cap that goes over the threads then whoever installed the filter had their eyes closed, or could have cut the cap off leaving the piece you found, possibly on purpose, if they were having trouble at work. It would not be the first time some malicious actions took place at a factory. Either way, you should be given piece of mind by LS and your dealer.