I think people need to read up on these chips.
Apple will refresh the rMBP lineup with Broadwell and Maxwell, and will wait until those chips are ready (I'm talking 20nm TSMC Maxwell here). There will be no resolution bump, A - it's unnecessary, B - it would be minor at best, C - Apple will be busy improving the displays for their Thunderbolt/iMac/Air lineup. It will perhaps be an upgrade to IGZO with IPS but at current resolutions.
The only difference between the 850M GTX and 860M GTX is clock speed, their architecture and core counts are the same. Remember Apple used overclocked 650M's and 750M's for their Ivy and Haswell rMBP's.
I believe Apple will probably use a slightly overclocked 850M GTX with 2GB GDDR5 for their top end rMBP. They will not want to increase the TDP of the chip, while with Maxwell on a new 20nm process (down from 28nm) they can keep the same TDP while massively boosting performance by around 60% max, so you are potentially talking (providing not CPU or RAM/VRAM bandwidth constrained) an FPS boost from 35FPS to 56FPS in a best case scenario which is fantastic!
Couple that with a cooler, less energy hungry Broadwell i7 chip and we will have one hell of a rMBP. I don't believe much else will change though, perhaps an IGZO IPS display will be used. Basically faster, cooler, longer battery life which is all good. I will definitely be upgrading simply for Maxwell as I do game a fair bit on my already very capable rMBP. Student discounts + solid resale value make it worth my while.
It all comes down to can Intel pump out enough Broadwell chips by late Q3/ early Q4 and can TSMC provide enough 20nm Maxwell chips in time (most likely). Intel is the big variable, they've had troubles with yields - who can blame them, at 14nm FINFET lithography is about as cutting edge a field as humanity does. So we will just have to be patient and wait and see.
Best case scenario at the end of October/ start of November we will get this upgrade, worst case we will have to wait one or two months more.