I have a genuine G60 and I wouldn't have another one. I have heard that if you want 16v upgrades, then you can fit a 16v 2.0 from a MKIII, and then fit your original 1.8 16v camshafts. The 2.0 unit produces 150BHP standard, but with 1.8 cams, the BHP increases somewhat.
Perhaps someone more knowledgeable about swaps could enlighten this response with figures/corrections.