I've been doing some more thinking on the wheels. In addition to the space out described below, I've been thinking some more on wheel diamter and tyre selection and come up with the following schematic.
I've also been having a rethink on wheel selection, still from Compomotive in the UK, but a three piece modular wheel design, as pictured. I like this wheel because the spokes are set in from the front of the wheel, giving it a more sporty, agressive look. I also think they fit the style of the Ghibli, with a nice simple five spoke design. I'd prefer not to have the studded rim (and a bit worried that it will leak), but this does have its advantages with respect to maintenance if the rims get curb damage.
I think I'll go with an offset of ET30. I could probably get away with less on the rear, but I'd like to have the same wheels on the front and back so that I can swap them round if the fronts get some minor damage.
The increased overall wheel diameter at the front should give quite some additional road clearance; about 20mm on the radius to lift the car to about the same as an MY94 model. The tyre selection will still give me about the same amount of rubber around the wheel as I have currently, so resistance to curb damage to the rim should be the same as I have now.
On the rear, the increase in diameter will give an increase in the wheel circumference and hence a bit of additional gearing. However, this is less than 10% and I know that the speedo on my Ghibli currently overestimates speed by about 10% (I measured it late last year when driving back from Germany; the speedo was reading about 265 km/h but my TomTom was reading about 250 km/h). So it's still legal.
For tyres, I see that Bridgestone offers the sizes that I'm after in their Potenza S-03 range (Bridgestone Tyre Size Line-Up), i.e. 225/40R18 on the front and 255/35R18 on the rear (and suitable for speeds over 300 km/h!).
I'm just running the numbers past Jorrit at Auto Forza to check if the clearances will be OK. If he thinks it's all OK then I'll be placing my order soon!