Is it worth going for a fully modular PSU?

It's not necessary from a performance standpoint, but if you're getting a case with a window it will look much cleaner.
All 3 of these are made by Seasonic-
http://pcpartpicker.com/parts/compare/antec-power-supply-hcg520m%2Cevga-power-supply-220gs0550v1%2Cxfx-power-supply-p1550gts3x/
 

Nuwan Fernando

Distinguished
If you could get a modular PSU, you can fit it with only the connectors you won't. But Non-modular means all the cables are already fit. So if you are going to use most of these cables, then the number of left-alone cables are negligible.But if you are going to use only few connectors, plenty to hide. So in cable management, it'll be easier for you to work with a modular one..

No performance impact.