You are maxed out, and I don't think it ever will be (reasonably) possible. I've read that the earlier non-Retina machines work with two of the new 16GB modules. And I am sure that the future models will eventually be offered with 32GB, if not at release (but who knows?). But our 2012-2015 rMBP's will never - read, never - have more RAM than the great Apple in the sky gave them at birth.
Upgrade friendly machines make for unpredictable consumer purchasing cycles and so are deprecated—according to the bean counters who had the bright idea to soldier in our RAM modules.
Ha - it's kind of frustrating in a sense because this machine replaced a late 2008 MBP 15 that started with 2 GB and got 4GB about a year later and then up to 8 about 3 years after that (along with SSD etc. etc.). Granted, 2GB was not the max available when it was purchased (I believe you could get 4GB at that point through Apple) and certainly it's not 16GB and definitely slower (1067mhz vs 1600mhz) so it's not exactly an apples to apples comparison on the purchase, but it's still fun to spec up a couple years later as demand increases.