is that stuff anti freeze as well? I will check with the local automotive store if they have something similar
in 70 they did not use an overflow bottle. how high would the coolant level be looking inside the rad? I suppose if you filled it all the way, it would throw some out when hot and maybe a couple times more, but that has to stop at some point right?
Thing is, I added a small overflow bottle out of some jap junker. filled up the rad all the way and the bottle to "cold" level. I thought it would throw out what wouldn fit in, but now when hot ,the bottle is filled to the cold level and when the engine is cold, level inside the rad is about 1" below the cap. I drove the car about 20 mls this morning and after cooling down the level inside the rad dropped a few mm again. could this have something to do with the bottle I use? maybe dripping from the overflow hose? can`t find any leaks or traces and it`s running great, not a single misfire, smoke or anything. No exessive pressure on hoses. I guess I`ll pull the plugs and see if they have a story to tell 
No, they are not anti-freezes. They contain a surfactant which breaks down the surface tension of the water which allows the water to make better "contact" with the water jacket and radiator tubes thus removing more heat from the engine and transfer it more fully to the radiator tubes and on to the fins. You ever see an ad for dishwasher soap that shows how it prevents water spots by creating a sheeting action of the water? That is a surfactant at work...keeps the water from beading. Then, they contain a lubricant package that not only prevents rust from forming but also keeps the waterpump seal lubed. Conventional antifreeze has a lube in it to do the same. Finally, they contain chemicals that prevent antifreeze from precipitating out of solution and plugging up the radiator tubes and heater core as it gets older. Modern antifreezes also include this in the forms that are advertised as "lifetime" or long life.
Water Wetter was the brand that was available a few years ago...it had a surfactant in it that improved heat transfer but a lot of us noted that rust would start forming after a few months if we did not buy a bottle of Prestone water pump lube and pour it in the radiator. It did not seem to have the other benefits.
My experience is that one of these products has lowered the coolant temp on hot days by about ten degrees when using either straight water or maybe a 20% antifreeze-80% water mix. YMMV. The ads usually claim more but there are a lot of variables involved. Depending on the product, about 8 oz works for a normal cooling system. When running a 50-50 Mix of antifreeze/water, I have not seen much difference and I suspect modern antifreezes may have some surfactant added trying to recover a bit of the heat absorption lost by using antifreeze.
As I said before, modern cars often run 225 degs and Porsche may not be concerned with higher than what we call normal operating temps as it is good for emissions if nothing else. High temps certainly help burn off vapors from the oil and such.
Now, non coolant recovery systems tend to run about 1.5" low, give or take. If you fill them to the top, they will soon blow some out as you said. If you use a coolant recovery system (as opposed to a simple overflow catch bottle), water is pushed into the bottle when the engine gets hot and then is sucked back into the radiator when it cools down. this means it has a pick up on the bottom of the tank to work and that pick up is covered at all times so it cannot suck air back into the radiator. The fluid is sucked back by a vacuum that forms in the top of the radiator as it cools. Fluid is allowed to be sucked back by a valve in the radiator cap that opens up as it cools down. This action will eventually remove almost all the air from the cooling system which allows it to work better. Air in the head cooling passages does not conduct heat well at all. Normally, the fill point has to be above the highest coolant point in the block in order to get all the air out and to fully fill the system. I think if you do a google you will find a better description that I just threw out.
An overflow bottle just keeps the blown out coolant from hitting the road and getting under your tires
At this point, I think you probably made an improvement by sealing off the air leaks, but I consider your clutch operation as questionable and I suspect your coolant is not optimized to run as cold as possible by removing as much heat as possible and transferring it to the radiator tubes.
