Could you please elaborate what this means re gains/changes or drops in performance please? Genuinely interested.
In the picture from the Naim Site, you see the transistor names on the PCB, 2SC3519 and 2SA1386, both types from Sanken. Those are NPN and PNP devices, giving a complementary output stage, unlike with all other naim amps, that use NPN devices only, in quasi-complementary fashion. Apparently they also made some other changes to the amp stage, to double the slew rate.
The first one to introduce a real complementary amp based on the classic RCA-(or Naim-) circuit was Les from Avondale Audio. His qudos NCC220 board have that too, and they outshine the predecessors NCC200 by far in direct comparison. It is actually much more modern, quasi complementary circuits have only been used decades ago, because PNP transistors were more difficult to make back then.
In general, this would mean increased performance, less distortion, etc. but it does not have to.
In case of the Avondale boards that introduced a complementary circuit, the feedback was overwhelmingly positive.
Eventually, the Nait is only a test-balloon, and the other amps will see this change as well, years from now...