The point is that "Custom" isn't a "little extra outlay". There was a time when a kit meant Asetek Waterchill and custom meant heatercores with home-made shrouds and Cascade blocks with Eheim pumps from aquarium shops. Those are no longer the best - you need an expensive Laing D5-vario or an Iwaki, you need a PA120.3, an expensive Nexxxos block or two. So custom now costs a lot to get the little extra performance over an Apex-Ultra with a D5, an Apogee, a Swiftech radiator and 7/16" tubing.
I should know - I've just replaced custom (genuine LRWW, heatercore, 1250, 1/2" Tygon) with a Swiftech 120.1, 6000 block, 1/2" OD tubing and an MCP350. Performance is very close, it fits easily in the case, it looks pretty and it cost less than the LRWW alone.