Why the need to go for the 5/8" ID tubing? While I can fit 5/8" barbs onto my block, I can't source the 5/8" Y's in Australia (yet). You aren't going to notice a significant increase in flow rates with a 5/8" system over a 1/2" system for a short CPU-only loop, that and 5/8" ID tubing is quite large and difficult to work with (bend, etc).
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