Apparently, the above link has been only causing further confusion.
Specifically, the laws governing denial of warranty coverage due to application of a non-OEM component are covered in the Magnuson Moss Warranty Act (15 U.S.C. section 2302(C))
In plain English; neither your dealership NOR Subaru Of America have the legal authority to determine nor decide whether your aftermarket component warrants a denial of warranty coverage.(c) Prohibition on conditions for written or implied warranty; waiver by Commission No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade, or corporate name; except that the prohibition of this subsection may be waived by the Commission if -
(1) the warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and
(2) the Commission finds that such a waiver is in the public interest. The Commission shall identify in the Federal Register, and permit public comment on, all applications for waiver of the prohibition of this subsection, and shall publish in the Federal Register its disposition of any such application, including the reasons therefor.
I found the following site that provides advice on how to proceed in the event of denial of warranty coverage which you believe to be unjustified.