Paul
I agree with your sentiments regarding manufacturers recommendations, however things move on, the P Zero rosso is a 10 year old design, it was considered one of the tyres to have back then, but there are better tyres out there today.
I have been trying to purchase the P Zero rosso rear tyres for my GS now for over a month and all of the reputable dealers could not get hold of it in the size, I checked the Pirelli web site and it is no longer listed as available for the GS, may be its being phased out? Which means we must make alternative choices, this is not always easy and the information from other members is very useful in making these choices, so is reading reviews and test reports from the the likes of Autocar and Evo. But as a general rule with tyres, you get what you pay for and the very cheap alternatives are just that and not up to the standard that most of us would expect on a Maserati.
Regards
Ascott