My favorite GT4, Mr. Waffles' wonderful Hyundai Genesis feels very comfortable at Mugello, but it lacks outright speed, being at the tail-end of the pack, along with the Guerilla Porsche Cayman (and the Rollovers' KTM X-Bow, which handled so badly that I never did get a fix on a reliable setup).
Also the Toyota Supra GT4: feels great, but lacking in alacrity. (We're splitting hairs here: the difference between the fastest and the slowest cars in this group was about a second...over a two-minute lap.) Great sound!
I had a problem with Shaun Clarke's new Ginetta GT4 (it sounded like a dirty carb jet or a loose plug wire), so I tried his older (2017) GT4, meant to compete with Kunos' GT4s, which had a different set of rules and were a fraction faster than today's GT4s. Looking forward to driving the new car when it gets out of the shop! Meanwhile, this is for the 2017 car.
The fastest of the lot was the Ferrari 488 GT4, whose developers admit that it was built to conform with Kunos' by-now-obsolete idea of a Balance of Power (BoP), not the new Guerilla/Rolllovers BoP standards. As usual, Maranello's driving experience beats everybody and everything else.