He outlined the components necessary in the OP, used Realworld data for the various parameters... it's up to the endusers to test the theory out in RW conditions. Anyone with a rig matching the original specification can check it all out by repiping from their current to any suggested tubing in the OP and validate their results against the theoretical results.

Rig Spec:
Laing DDC1+ (unmodified)
Thermochill PA120.2 with 2 x Yate-Loon fans at 12v
Swiftech Apogee GTX
Conroe C2D CPU, overclocked and under load, emitting 100W of heat
2 meters of tubing length
Change current tubing over to any of the suggested (other than current, obviously)

6.35 (1/4") ID tubing with quick-fit fittings
8mm (5/16") ID tubing over 6mmID|8mmOD barbs
8mm (5/16") ID tubing with quick-fit fittings
9.6mm (3/8") ID tubing over 7.5mmID|3/8"OD barbs
9.6mm (3/8") ID tubing with quick-fit fittings
11.1mm (7/16") ID tubing stretched over 10.5mmID|1/2"OD barbs
12.7mm (1/2") ID tubing over 10.5mmID|1/2"OD barbs
Then validate against

Final CPU temperature is ambient (22C) + system load (114W) * radiator C/W + CPU Load (100W) * block C/W

The final CPU temperatures work out to be:

6.35mm quick fit = 34.21C
8mm barbed = 34.08C
8mm quick fit = 33.91C
9.6mm barbed = 33.89C
9.6mm quick fit = 33.80C
11.1mm barbed = 33.79C
12.7mm barbed = 33.77C
Can't expect Cathar to do ALL the work - he's given everyone the foundation to decide for themselves, and the necessary info to validate for themselves (obviously, account for change in ambient if one exists)... time will provide the realworld-condition tests.