I love math problems, so I'll calculate it for you:
if 5 gpm in 1/2 tubing is 8 fps then... (mumble)...
2.5 gpm in 1/2 tubing is 4 fps!
So you're all set
Increasing the tube size from 1/2 ID to 5/8 ID will not yield an appreciable increase in flow rate/speed, and certainly will be immeasurable in CPU temp. If you want the actual calculation, let me know.