Neale,
You're absolutely right about not needing a sophisticated set-up for cooling. We've seen people using radiators and fans normally used for PC cooling which is unecessary, but how are you supposed to know? I've seen no information from the spindle manufacturer on required water flow and the only clue you get is looking at the rough size of the pumps provided in the all-included kits Huan Yang sell.
What made me decide to replace the 5mm tube with 8mm was a test of the flow rate with and without the long length of tube. The pump capacity was just being wasted and I wasn't confident that 30l/min would be enough. I documented this elsewhere on the 'Plumbing a water cooled spindle' thread.

One thing that did surprise me was the difficulty in finding a small enough barb to fit to the pump, especially when the pump box didn't specify it's thread size. My local hardware store helped me identify the thread and we found a fitting, but I had to modify it in my lathe to make it fit. One of the plastic fittings in the pump box was just right for the 8mm pipe and I'd recommend going down that route just to avoid the problems I had. Those of you who live a little nearer to the centres of civilisation may well have less of a problem though.

Kit