I spoke with Modena Tec (who makes wheel spacers/ studs for Coupe). He said don't do it if you are interested in performance. These items are strictly cosmetic. The same sentiment has been echoed by other mechanics that I have asked.
I think, putting spacers on the car will change characteristic of the car but dont know how much it will effect. Btw, cosmetically, it look nicer than stock. Too thick spacers may lead to premature wear/fail on the suspension parts. Eibach and H&R made spacers for many Ferrari models which also will fit our Maseratis. Actually I am looking to buy H&R 15mm spacers. They cost about $150/pair.
Agreed that spacer is just for cosmetic purpose, my brother had it on his Porsche 996 with a wider tire, the performance is down a little bit and couple months later he found there are uneven tire wear on the inside tire wall. He now removed the spacer and feel happy with the original setting. I think that's the cheapest way to make your car looked better but have to sacrifice with performance. For your own choice.