Well, the presence of a certain number of Porsches in a leaked dev build doesn't mean that a) we will be getting Porsches in FH3; b) they will be all the Porsches we'll be getting.
I doubt T10 will let newcomers to this whole "having Porsches in our games" business make a mockery of them. A Horizon 3 Porsche pack would likely include a generous helping of the latest road-going machinery from Porsche, with a side of iconic vintage 911s and pre-911 cars. And I wouldn't be too surprised to see them abandon the "Porsche Expansion Pack" gimmick in favor of one (or several) "booster pack(s)", a la Kunos.
But I believe Microsoft's guys will be saving the real party trick for Forza 7, where we may see different permutations of the 917 go up in arms against late 60s prototypes and early 70s Can Am monsters, the 935 dominate the IMSA GTX field, and perhaps even the 718 or 804 F1 cars compete in a new F2-ish GP group against the Ferrari 158 and God-knows-what-else.
Of course modern race cars and other, more esoteric retro racers would have to be featured, too. All would be in line with the strategy that T10 employed during the transition between Forza Motorsport 5 (where DLC cars explored some new racing categories, such as early 3-litre F1 cars and Can-Am racers) and Forza Motorsport 6 (where said categories were expanded at day one already with the addition of cars like the AAR Eagle and McLaren M2 in the case of the former, and the Lola-Simonitz and McLaren M8 in the latter).