Thanks for the advice everyone, you've all been a great help.
I fired up the DK2 for the first time last night using the Zotac 980ti. Initially I was surprised it didn't look too different to the 970 but I raised the ss to 1.5 and it settled down to be quite stunning. I didn't check the FPS but in space and flying around the outside of a Coriolis I could not pick up any drop-off or judder whatsoever. The improvement in quality was incredible and very immersive, to the point where I can see myself playing ED way more with the rift than without now. Landing was entirely different though - it struggled a bit on planetary approach and using the SRV. I'll have to find some simple setting modifications to apply when I go driving. I'm also getting a bit of that 3d anaglyph issue that I figured was due to my IPD being measured by the rift configuration utility - I'll try some other measurement techniques and see how that goes.
Set up:
i7 4790 stock
Zotac GTX980ti stock
16gb DDR3
120gb SSD
Settings on high with 1.5ss and I did not touch the Nvidia control panel.
Light blue HUD (green is nasty!).
Total cost (not including monitor, DK2, kb+m, Win 7, HOTAS etc) was £690 for the PC, £520 for the 980ti, £70 for the SSD and ram upgrade, and I'll be selling the 970 for £200 inc. postage if anyone is interested? Shameless plug to Freshtech solutions for all of it including great aftersales.