This thread is an interesting read.
I have been looking to replace my PSU for some time and all but settled on the 650w Supernova G2 - the Seasonic units have really piqued my interest now though, noticed the 550w Focus Plus Platinum is on TOW for £89.99 and I'm very tempted.
Not sure that 550w is quite enough for my system though? Most places seem to recommend a minimum of 650w for the 780Ti...
550W is ample. About 250W-300W depending on the 780Ti model. 350-400W total system power draw in most website benchmarks. Although you could go for a 650W to run something like a 1080Ti or Vega more comfy, in future. This one's on offer too:
My basket at Overclockers UK: