The new 850 and 860m are a very substantial move forward. If you review the NVidia profile pages on these two cards, they list the Futuremark performance numbers.
On the Geforce 650/750M cards, I was seeing a Vantage score of about 11,000. The 850M is now scoring around 16,500. So that is indeed 50% all by itself, and almost precisely what the previous gen 765M was scoring in an Alienware 17. The 860M on the other hand has blown past the 765m and is scoring about 20,000 putting it closer to the previous generations 770m (about 8 percent short.) The 860M would be close to double the 750m based on these statistics.
As Apple's last update was in October, I do believe we will see them spec bump the Retina in April, because thats what they tend to do...two updates to the unit per year for the last 6 plus years. They have also had updates of just the video cards.
I would bet on an 850m, but this time I'm actually undecided on what Apple is going to do. I don't think they care about gaming much (if at all) but they DO care about general user experience. And with the native screens pushing so many pixels, and 4K displays almost certainly coming this year I think they might evaluate hotter cards.
Folks have listed the 870m in the Razer 14, but take a look at the latest MSI GS70 as well. This machine is thinner then both the Razer and the MBP and they put the 870m in that machine as well. TDP won't be as much of an issue as power draw draw will be. I am wondering if Apple could make a 100w PS that isn't noticeably larger then their 85W.
See the performance graph here.