I've just researched the tyre issue. I just bought a set of Yokohama V550 245/40R18's. They are the same size spec as the OE Dunlops but they seem to have a wider tread footprint.
It seems there are some variations between tyre brands and particular tyres as to the actual width of a tyre - the size on the tyre (245) does not necessarily mean the exact tread width in each case.
The Yokohamas have been on for a few days so I haven't really tested them out fully. They are much quieter than the Dunlops and the ride is smoother. I'm happy so far and they are noticeably (to me at least) wider than the old SP9000 tyres.
I seem to remember reading about someone putting 265/35R18's on the back of a new GT but I can't see how they would have got them on an 18"x8" rim. For example, the Yokohama site states the rim width size for 265/35R18 is 9"-10.5".
In Qld the DOT states:
"The rims and tyres must not protrude beyond the
bodywork of the vehicle, including flares, when viewed
from above with the wheels facing straight ahead. If the
vehicle was originally constructed with a portion of the
wheel protruding, the wheels must not protrude further
than originally constructed.
The tyre to rim fitting and the tyre to rim combination
must be in accordance with the Tyre and Rim Standards
Manual published by the Tyre and Rim Association of
Australia. Reputable tyre retailers should have this
information and be able to advise on the correct
All rims fitted to an axle must be of the same diameter,
width and offset. They must not have a circumferential
weld other than that which attaches the outer rim to
the centre. All rims must have stud hole pitch circle
diameters suitable to the hub. Wheel nut tapers must
be appropriate to the wheel and must engage the thread
of the wheel studs for at least the same length as the
nuts provided by the vehicle manufacturer.
Slotted and elongated stud holes are not permitted.
The maximum tyre width for a car or car derivative must
not be more than 1.3 times the vehicle manufacturer’s
widest optional tyre."
I'd check your local tyre shop for further info. They are the experts.