I'm pretty sure it was resolved. Here's a portion of an e-mail from Tadge dated May 2015.
Although there are very few complaints from Z51 customers on cooling issues, we are looking at taking some of the learnings and hardware from the Z06 and making them available on the Stingray. For example, the front-mounted supplemental trans cooler developed for the Z06 automatic will be included in the Z51 package for automatic coupes starting in the 2016 model year.