I want to POR 15 some engine tin and other bits, and I'm
wondering if it's totally necessary to use the POR 15 primer before
painting over them.

From my experience with the stuff-no. I used it to paint the tin on my '77, and never top coated it with anything. Since it's not exposed to sunlight it held up just fine. I've also topcoated POR 15 with any good quality primer, then covered with top coat of automotive paint and had no problems. Their claim that you have to use their primer is just to sell you more of their products, IMHO.  Jeff