I would think the more pressure the better but there must be a limit for some weak link. What would the weak link be? Hoses and heat exchangers come to mind but they should be happy w at least 15psi.
With the potential for Lehman engines to overheat on the back cylinder more pressure would be an asset keeping the coolant from boiling. The combination of high pressure and high coolant temps could benifit fuel burn as well.
North Western Washington State USA