The 'performance image option' is something I knew about in the 70's, but was not sure what it was. I read someplace (a long, long time ago) that you could get the trumpets as part of that option prior to the GT. What I read may have been the same parts info Mannel refers to. My Jan 65 kar has trumpets but I have no way of knowing how they got there. I know they had to be put on by the 1st owner or the dealer if not factory. They were on the car in 68 according to the 2nd owner.
There was also a prior post that showed an E-bay auction with "ARVIN" option trumpets that had the trumpets and a pattern to cut out the valance.
Bob Mannel posted this in a previous discussion about dual exhaust.
"The April 1965 Ford Master Parts catalog was the one that listed a "Performance Image Option". The system listed was the Arvinode system with the regular H-pipe and trumpet tips. I am not sure if this was just an early name for the GT or not. Ford gave a date of 3-22-66 in its parts catalog for elimination of the Arvinode system, but a date of 4-2-65 for Metuchen and Dearborn, and 4-4-65 for San Jose in its Technical Service Bulletin Article #315. After those dates, the Arvinode front glass-pack was replaced with an oval muffler. I have not heard of a GT with the Arvinode system. Maybe someone here has. The elimination of the Arvinode and the introduction of the GT occurred about the same time.
According to the Ford parts catalog, there were many Mustang dual exhaust systems. The 289 HiPo was the first with a transverse muffler until October 64. Then the Arvinode system came out for the HiPo. There was also an option (called a "Performance Image Option") for the A-code which was the Arvinode system. The Arvinode system was dumped in favor of oval mufflers in April 1965. The GT came out duals as well. By the end of 1965 you had the 289 HiPo, GT (All A-codes), and optional duals for A-codes. HiPos had the unique H-pipe to connect to the HiPo exhaust manfolds, and the GT had the trumpet tips. Performance Image Option also shows trumpets. Looks pretty similar for 1966.
What is needed to really clear this up is a dealer options book for 1965 or 66. Must be one somewhere."