Don't base the 950m performance just on specs alone. Maxwell is a very different architecture than Kepler that's used in the prior 750m chip. On real world gaming benchmarks, the 950m smokes the hd8870m that's basically the same chip as the rebadged m370x. In most gaming benchmarks it's 30-40% faster which is huge on the same 50 watt power envelope as the m370x. Even with compute, maxwell is much faster than Kepler and close to the performance of amd. So basically either AMD is giving these 3 year old chips for almost free or Apple is retaliating against that stupid nvidia lawsuit.
I think the saddest thing of all is in the last 6-7 years of MacBook pros, Apple has put in the best gpu they could based on heat/size constraints of the time. This time Apple went with the cheapest or most politically expedient choice.
In all fairness Maxwell is updated Kepler architecture. It has just bigger internal caches and lacks DP engines, thats why its so efficient.
Also, I believe Apple went with GCN architecture because its more mature in fact more efficient than any Nvidia Arch(Green500 list says that GCN is the most power efficient GPU arch. that has ever been on this planet, and lets look how big performance Apple got from 129W of power on Tahiti chip in Mac Pro) and will last longer compared to any Nvidia Architecture.
Lets look how fast Kepler arch gets old and outdated in games(Witcher 3, Project Cars). GCN with time gets more and more performance, which is not and will never be the case of any Nvidia arch(The way its meant to be milked...).
Also the factor of OpenCL performance is biggest thing and most important for Apple. Not to mention Mantle/Vulkan, which i believe soon will be flagship API for OSX.