Nice question. I'm planning to upgrade my stock rims to 19" Iforged Aero, but I havent found a dealer that will check if it fits. I've called RDM wheels and they said the Iforged guys could actually customize everything to fit the wheels to whatever offset, bolt pattern we may have. I live in Florida, not much around here. I'm still searching..
The 19" rims fit because I called HRE and the sent me several pictures of Maserati's with 19 and 20" rims. The 20" rims had very thin rubber on the rear. I ended up not buying HRE wheels when I saw the OEM 18" 15 spoke wheels redone in black chrome. It looks great on my gray car and I saved $4k vs HRE wheels. The wheels were done on an exchange for $880 at Coast General tire in SoCal. They are one block from Orange County Ferrari/Maserati and do a lot of work for the dealership. Ask for Ron.
Also, the dealership did not know the offsets but HRE did. I have the offsets written down somewhere and I'll post them soon. I was looking for a more aggressive wheel (with a lip) and thought the OZ Nova monobloc used on the late model Lotus Esprit Turbos would look good. Unfortunately, they are discontinued and I couldn't find a set.
I'll try to post pictures from HRE as well as my own car but the info I have from HRE is that they use 49mm offset in front and 43mm offset in the rear on 19" rims they build. They use 19x8.5 in front and 19x10 in the rear for their HRE wheels. The Maser dealership has told me that they have had a lot of aftermarket wheels "vibrate". The only ones that have worked well have been from HRE. When I talked to HRE about it, they said make sure the wheels are "hub centric". Hope this helps those considering upgrading their wheels. I just set up a website at www.maseraticc.com