It's partly this age-of-the-rubber issue that is making me consider alternatives - including the similar but smaller width equivalent wheel from certain Mercedes later SLK's - narrower but almost the same circumference and weight limit.I must stress the main reason I went with the Merc wheel is the same size tyre and rim diameter.
I don't think anything less than 17" will clear the brakes, and I didn't want to risk any tyre that is smaller in rolling circumference that could cause problems.
It is over 5 years now since I made the kit up, and it is a valid point regarding age, I recall one instance of the space saver (not sure if Merc or OEM) being inflated and found to be 'rotted'.
I've never used me, but my FD rims stick out right to the very outer lip of the front wings with no issues.
Can one buy a new (= just manufactured) proper sized Space Master tyre (as per your set-up and the Maserati original) - and if so from a normal tyre depot or only from Mercedes or Maserati ? This would remove this worry. As an aside, I have (since new) a 1982 Renault Alpine A310 V6 that also came with an inflatable spare which I have never had to use and which I would be frigthened to use...