Tony,
Straight (distilled) water is a far better heat transfer agent than is coolant (ethylene glycol or similar compounds).
I ran my 89 SHO for a whole summer on nothing but water the first year I had it as I was constantly working on trying to revive the cooling system and to repair its many problems that it had when I obtained it. Straight water is an excellant coolant for summertime use all except for those pesky issues of corrosion and pump lubrication. Water Wetter from Redline is a good supplement for those cooling systems to overcome those issues.
Running straight coolant (undiluted) would cause the engine to run slightly hotter since it's a relatively lousy heat transfer agent. However, it won't cause an overhieating problem if the rest of the system is operating correctly,
The pressure in the system is one of the more important issues that gets overlooked. As long as the system stays pressurized, the boiling point of the coolant (whatever it is) is raised substantially and it continues to transfer heat as well as it can.
Steve
