Here is the quick info ... I'll make another thread with pictures and all the details and link here.
Album on Flickr with all the pictures: https://www.flickr.com/photos/surfergeek/albums/72157660194695686
Short version:
I ordered Forgestar CF5 wheels 18"X11" +52mm Offset
Wrapped them in NTO1 305/35R18 Tires.
I have a square set-up that fits front and rear, no problems with clearances in the rear (sway bars, end links, control arm relocation brackets, all plenty of clearance).
I have Cortex Racing Offset for 315 tire coilovers in the front. With no spacer, the rim goes on in front, but it's very close to the strut.
You can run the Forgestar center caps that come stock with the wheels, they clear the hub centers in the front with this much offset.
With a 5mm spacer, it has plenty of clearance in all aspects and that could be run.
I'll be running an 8mm hub centric spacer, it gives me a bit more turning radius / scrub radius and keeps me in the factory scrub radius and gives me a touch more clearance for my brake cooling ducts.
Requirements:
Extended Length Studs
I have about -1.8 of Camber and looks like no fender rolling will be required
Steering stops probably a precaution to take just a little out if you have brake cooling ducts or the like. (I'll be putting in a set of Roush stops as a precaution)
Here is the great part that we found today.
Test fit on a 2010 GT on Koni struts/springs with Brembo 15" 6 piston brakes in the front.
1. CF5 in this offset has tons of clearance around the 6 piston calipers. You can run a 15" brake set-up and still run an 18" wheel with these wheels.
2. You can still run an 11" square set-up even on struts springs, but likely would want a slightly different offset.
In our test fitting, 5mm of spacer wasn't enough (Rim contacted the strut).
A 10mm spacer was very close to getting it done but still too close.
A 15mm spacer gave adequate clearance.
For someone without the Cortex offset strut housing / struts springs it looks like you could go with a +55mm offset (be fine in back) and run 8-10mm of spacer in the front. If you are comfortable with 15mm of spacer in the front, the +52mm offset would work for you too.
A quick plug for Auto RnD (source for my forgestar wheels) Rishie (owner) is a great guy and very knowledgeable when it comes to wheels and suspension set-ups. I have done business with him for 15 years and he is a standup guy. He also can source Enkei's in prime fitments or just about anything else you could want.
His site: http://www.autornd.com/ Give him a call.