hi gakusya, i think i got the same problem as you got.
A WRX STI 2006 2.5L + atp3071 turbo
Coolant overflows when i drive hard (i think it's more after driving hard)
No overheat, No consumption (oil or coolant), No milk, No smoke
A few little black things (2 or 3 in the bootle)
Tryed with a 1.3b upper cap => still overflow
changed my radiator with a koyo => a little less oveflow
changed the termostat with a crucial => still overflow
Coolant has difficulty to go back into the radiator except by very low temperature.
I did the carbon test (blue/yellow test) => No carbon, fresh air.
My compressions are balanced and ok.
My sparks are ok.
My coolant water is a common elf coolant water.
Can you give details about the coolant you put (brand, specs...). Thanks