Just to share an update on my PSU effort from last year (posted above) to power both Pi and Screen off one plug - finally I got chance to improve this. I was getting the odd 'rainbow' warning where the voltage must have just been dropping a little too much on occasion. I silenced it as it was only popping up maybe once or twice per session, but always figured I should look at it one day.
So in an attempt to solve the problem, I completely did away with all molex adapters, used a different USB cable which only carried power (no data - just more convenient when stripping cables), and shortened the USB cable by well over half to reduce any unnecessary resistance. On the last effort I was overly conservative with cable length. I then had a friend help me solder each wire together, heatshrink the individual wires and then put one big piece over the lot. Looks a lot cleaner now and the connection is obviously more solid. Here is the result:
Most importantly, no more rainbow so I now have full confidence in the cabling. Doing away with the molex, shortening the cable and directly connecting everything has obviously helped.
Just thought i'd share.