The fronts are 275/35-18s, which are 0.9% taller than our 265s.
The rears are 325/30-19s, which are 2.1% taller than our 295s.
These diamters will definitly fit in the C5's wheel wells. The front/rear delta from stock is 1.2%, which should not effect traction control or active handling.
I have 315/30-18s on the back of my Z. A 325 is only 10mm or about 0.4" wider than my 315. Split even in/out (it's 0.2" wider either side of the centerline for a total of 0.4"), the 325 takes up less than one additional quarter inch more space on the inside than my 315s. My 315s have about 3/4" of clearance on the inside, so a 325 should probably fit with over 1/2 inch of clearance. Without tubbing.
The only question would be the offset of the C6 wheels. But the tires themselves should work.