While raw water cooling has an unlimited supply of cool water, a closed system is pressurized, which prevents localized boiling in hotter parts of the system (cyl heads, valve seats) so if the system is designed with enough cooling capacity, it should provide improved cooling. It also allows the engine to run at at steady temp, closer to what GM intended these engines to run at.
It only makes sense if the engine has no flaking rust inside, otherwise that will clog the heat exchanger.
