lincolnmk5,
Within the last month I purchased a mk 5 GTi, also with 18" Monza II's, and am also with Admiral. Irrespective of whether the car left the factory with them on, they are in fact a modification in the insurers eyes, as they are over and above the standard spec. Fancy wheels make the car more attractive to potential thiefs, therefore more risk, therefore more premium. Because I am partly through my insurance year, don't know the exact uplift paid, but probably about 10%.
The bad news of course is that when you claim for the loss of your car including the new wheels, the insurer will only pay out on the standard cost of the car.
Please don't shoot the messenger.
Just on Admiral, I previously insured an Integra Type R with them for 9 years, and they were always the best price I could find.
Hope this helps
KB