The reasonable explanation is that the added convection efficiency due to higher flows (beyond what you can get with pond pumps) results in only marginal improvements. Look at blocks such as Cathar's and you'll soon see that thermal resistance of the TIM joint is often the single largest resistance in the system. This is true at flows under 10 l/m. Added pressure generally brings two things along with it, noise and increased potential for leaks.
Take a look at C/W values vs flow and you'll see that they all flatten out long before the 10 psig range.
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