When I bought my 2.8 Japanese cup, I did contact M Collina to have more informations about my car and he said that there was 27 Japanese 2.8 cup.
I quote him :
This car was one of the 27 Ghibli Cup produced with 2.790 cm3 engine type AM574 as requested at that time by the Maserati Importer in Japan.
The engine was the same that the one in the 2.8 ghibli but there was a different file in the ECU allowing for more boost.
By increasing the boost pressure, the car demonstrate 305ps@6100rpm and 43.5mkg@3650rpm.
Also, the final drive ratio is lowered by about 6% (3.77 vs 3.91), this one made a big difference.
I think that 3 RHD 2.8 were converted to cup specification in UK, but I don't think it was done by the factory.
Having driven both (2.0 and 2.8 cup), the 25HP difference in the last 900 rpm can't compete with the 30-50hp difference from 1500 to 6000rpm in favor of the tweaked 2.8 (my probably biased opinion).
I don't have the power curve from the tweaked 2.8 but you can compare the Cup curve with the standard 2.8 and add about 20-30HP everywhere to have an idea.
Do you know if the final ratio of the 2.0 cup was lowered too?