How to change the camshaft V6 engine is not very popular / described in the network. The fastest is to insert the inlet cams from 3.2 GTA, after remap you can get around 20KM growth. Inlet cams used in the GTA are approximately 246 degrees duration, stock cams in 2.5 and 3.0 engines used in the AR156 and AR166, have 226 degrees duration, 9.3 mm lift.
Personally, I have custom-made set of camshaft 278 degrees about the times and lift of 10.4 mm, I think that within a month in the end I find some time to piece together the engine then I could show you how much more power can we make after change the cams and a remap. It is true that I have in my 156 engine 3.0 but 2.5 will react very similarly as to the scale of growth performance.
Alfa Romeo 156 3.0 V6 24V 231KM@280Nm