I am all for twin turbo 911s. Just to keep the dream alive!...I don't know the details of smog testing (or whatever its actually called), or if there's some kind of "family" strategy that can be used. In any event, I think that would be tiny compared to the R&D and tooling.
My point is that Porsche didn't develop a pair of turbo fours as a one off for the 718s. They developed an engine family that could cover from at least 2.0L single turbo fours up to at least 3.75L (2.5 * 1.5) twin turbo sixes. Productizing a variant incurs some incremental cost, but the overall cost of all this should be considered spread across 718s and 911s.