Many enthusiasts don't know Abarth. That, I think, is a real issue for Fiat. Yes, 95%+ of the car buying public doesn't matter to Abarth, but for the 5% (or whatever) that does, Abarth is still largely unknown. I am not sure this is the best first step to reach that market. The car in the house is great. Just not sure about using Sheen. Is a guy who has been caught driving drunk going to appeal to car guys? I think not.
Also, I see the spokesman issue differently in this case. Wheaties can survive Phelps because that brand is bigger than one person. At this point, in the USA, Sheen isn't the guy in the Abarth commercial, Abarth is the company with Sheen in their commercial. Sheen is bigger than Abarth. That is why I see this being a bigger risk than the normal spokesman association.
I hope I am wrong and Abarth flourishes. More enthusiast friendly cars is always a win.