The dealers pretty much said that there is nothing they can do as the car is working fine. Just follow the bulletins which tells you to shift earlier... as for parts there is no current GM part to improve the cooling, its only for regular stingrays to improve their fan into a Z06 fan.
On a side note, I spoke with someone whose friend also tracks their 2015 z06, he also had overheating issues and power steering issues. His solution was putting in plastic spacers to increase space for the piece of plastic above the cooler. Apparently he hasn't had issues with heating afterwards and hes setting some good track times (so he must be pushing his car).
As for how that is done I have no idea have to bring it to a shop and ask them about it as I am not familiar with the parts.
- does this void the warranty?
Also over heating issues occurs much more for the automatics only, in the 17s they made improvements for only the manuals i think.