Ok so if I understand you right, let the car warm up fully first (e.g. 92C+) so that all the coolant the cap needs to release when engine is hot would have been dumped out into the overflow, then hook up the overflow hose so that it's pointed at the windshield, and go drive it hard and see if it sprays when boosting?

 

I should also probably add that there are no obvious signs of coolant being pushed out from the overflow. The level rises maybe an inch and a half when fully warmed up, but is still far from the top of the tank. The underneath of the plastic yellow cap is always dry, and there are no stains around the edges (which should be obvious with the fluorescent dye).