I'm not sure if it's 10% - got 19 GPPs (each of the 5 types) for an 86 production project, 20-22%. Online speed if it makes a difference.
So you get more overall GPPs from Carnival, but fewer on any one type (50% of the Engineer or Merchant points you would get just running the Industrial Zone Logistics or Commerce Hub project). If you're pushing to get a specific person, a la Adam Smith, Carnival is worse.
There's synergy with the 20% GPP recovery bonus to spam Carnival projects, but overall it doesn't seem great. Someday I'm going to take the Minas Gerais for a spin though.