Creme :
We're talking about DX10 cards that can't run modern DX11 games anyway, but at least get support for any problems until April 2016. AMD dropped support for DX11 cards that could easily play games in this day and age. Nvidia still releases updates and game specific drivers for 400 and 500 series.
It's competitive because of HBM, which lets them brute force and cram more shaders.
The DirectX version has nothing to do with it and Nvidia doesn't develop specific drivers for the 400 and 500 series cards. The 400 and 500 series, as I've said many times now, use the same drivers as the 600, 700, and 900 series because they have the same basic cores (among other reasons). Developing drivers for both VLIW5/4 and GCN would be like trying to develop drivers for both an Itanium CPU and a Skylake CPU at the same time to run the same software.
HBM doesn't make that big of a difference. Yes, it helps, but it is far from everything, otherwise the Fury and the Nano (which have fairly similar performance) wouldn't have such a huge difference in power consumption. The biggest improvement in power efficiency was fixing powertune. We can also note how the 390X/290X cards and the Fury/Fury X don't have such a big disparity in power efficiency, so although HBM uses a lot less power than similarly performing GDDR5, the power consumption of the memory is still far from being the majority of a card's power consumption. HBM allowed for higher memory performance more than it reduced overall power consumption of the card.