CPU with 1/2", NB/GPU with 1/4" paralell via Y Connector?
 

Well, the whole question is in my title.

I want to build a CPU waterblock with the 1/2" System, but there is not much space at the NB / GPU waterblock for this system.

Will I have got a lack of performance, if I have the 1/2" System on the CPU Waterblock, the EHEIM 1250 and on the radiator, and have the 1/4" System on NB /GPU where those two waterblocks are paralell in the System via two Y connectors?

It's certainly not optimal. Consider that the 1/4" barbs will probably have an ID of 1/8". For the 1/2" barb the ID is around 3/8", so you just restricted flow by another 33%. If at all possible, I would try using a waterblock with a single inlet and two outlets (the whitewater is an example) and then use 3/8" hose for the runs from the two outlets to the GPU and NB. This should give you a fair bit more space. You can also take a drill and widen the ID of the hose barbs to increase flow rates.

Actually, the tubes i have in mind for the 1/2" have got 10mm ID and the 1/4" have got an ID of 6mm.

I think this should work, don't you?

I'd use 10mm copper tubing for the NB & GPU barbs, The ID is the same as a 'comercial' 1/2 barb (9~10mm) but you can use 3/8inch tubing which fit's much better!. The only downside is you'll have to make your own Y splitter that does 1/2 inch to 2x 3/8. I'm going to use 13mm & 10mm Cu tube to make mine cause plastic ones are VERY restrictive...

I'd have the GPU & NB running parallel after the CPU like this:

If you have a two outlet block (White Water) you can dispense with the first Y splitter, just make sure you balance the flow through the NB & GPU so they match headloss/restriction...

(20 edits later!!)

My CPU WB has an 1/2in inlet(thinking about upping it to 3/4 to fit my johnson) and 4 3/8 outlets. 2 of the outlets run to my HD cooler, 1 to the NB and 1 will go to ther GPU. If your making your own CPU block then consider having multiple small outlets instead of a Y splitter.

I assume you need the 1/4 tubing because your NB/GPU blcks are small. Have you considered running running 2 inlets to each? Each fitting won't take up much space so you may have room to add 4 inlets(2in/2out).

Actually the 1/2" system would have enough space on the NB waterblock, but I designed the GPU waterblock with inlets on the sides of the block in order not to lose too many PCI Slots.

As for that the version with 2 inlets and outlets sounds handy, but there isn't enough space either. (with 1/2" System the GPU waterblock would have 4-5cm height)