i have a simialar problem in my R32 GTS4 - no performance loss, but after a drive in the hot wheather after i turn the car off the coolant overflow tank and almost full and is making a bubbling noise. they is not leaks, not smaoke out of the exhaust, temp gauge doesn't go up at all, stays at halfway. i also had a water pump replaced about 4 - 5 months ago and the guys only put water back in, about a week later i drained the radiator and filled it with coolant. could the problem just be that the water is overheating.
p.s. colour of water is green and no signs of oil or anything. also no water in engine oil.
any help would be great