My opinion to the following cars compared to the Z/28, obviously we don't know what track it is "3 seconds faster" than the ZL1 at BUT. This is what I think.
ZL1 - Z/28 should be easily faster, if its slower than this anywhere it fails ( in my opinion ), other than a drag race maybe
GT-R - actually a faster car than it is given credit for, BUT being a track based car the Z/28 should be fairly faster
911 GT3 RS - hard to say, based on the times I know of and find on this car its actually not as fast as many think. I don't live on the track so I don't see daily runs of this car BUT to my own knowledge I'd say the Z/28 will actually be faster
458 - Similar to the GT3 RS, I don't think the 458 is as fast as it should be, on most tracks the Aventador is several seconds faster and the Aventador is a Lamborghini... Generally Lamborghini's like speed not corners. (Aventador is an exception I think). That is another discussion I think though, so I'd say the Z/28 should have that in the bag as well, especially being so track oriented. However Ferrari owners don't really track their cars daily.... they prefer to let it garage sit... sadly...
Z06/Z07 - This actually I think will be the hardest competition I'd say it'd be VERY hard for the Z/28 to out do the Z06/Z07 on a track. The Z06/Z06 and even ZR1 are not given a proper amount of credit for their track capabilities. The Z06 out of these cars weighs the least, has the best torque figures, Best weight distribution the GT3RS and 458 are back heavy and the GT-R and ZL1 are front heavy, I'd say out of the 5 this would be the hardest for the Z/28 to beat.
Just my opinion