The 7AT on the Z is not a DCT, but so far I'll say 90%+ people who has tried or drive/own it praise the gearbox very much, fast and crisp.
The acceleration time is also comparable to 6MT, so why would NISSAN bother to put in a DCT while the regular 7AT is capable to do such a good job.
And if it is the case, it will be even less wise to put in the DCT since the price of the Z will have to be higher thus hurt the already low sales number.
CAD$0.02