I spoke with Jorrit yesterday and got some further insights into the Maserati Ghibli!
It seems that on the MY92 cars, Maserati left the lip on the inside edge of the wheel arches unfinished, so these stick out into the wheel arch at a 90 degree angle to the body panel and impair on clearance of the wheel. On the MY94 cars, presumably as a result of factory fitting larger wheels, this lip was rolled out on the top half of the wheel arch, increasing clearance between the body panel and the wheel.
As a result, on my MY92 Ghibli, I'm a bit more restricted on the offset that I can fit before I start getting interference between the wheel and the body panel. This means that the minimum offset on the MY92 cars is restricted by about 10-15mm from the MY94 cars, unless the lip is rolled up. Since this modification will necessitate a trip to the body shop (to get the metal resprayed after rolling it up) then I have decided limit the reduction in offset.
I also discovered that on the MY92 cars, the offset of the factory wheels (the seven spoke OZ Racing wheels) is ET35 on the front and ET32 on the back and not ET35 all around as I originally thought.
I enquired with Compomotive the prices of the MO and PC wheels, but they referred me to a local retailer, so I'm planning on making a visit to get the wheels properly measure up to ascertain the optimum offset specifications. They did however send me a nice picture of some 18" MO wheels fitted to a Ferrari.
These are 9" wide on the front and 10" wide on the back. I find them a bit delicate, which suits the Ferari, but not sure it fits the more agressive styling of the Ghibli. So I'll probably go with the PC wheels still.
For info, the Dutch retailer for the Compomotive wheels is Korst Banden BV.