It seems the coolant is overheating when being driven hard, which it shouldn't do.
The coolant in the radiator will expand a bit when hot, but it should merely expand into the expansion tank until the rad temp cools, when it's sucked back into the rad again. There shouldn't be enough for it to throw some out of the exp tank cap. When cold the level in the exp tank should be somewhere between the min and max marks, probably towards the lower level.
Causes for overheating can include a faulty or blocked radiator, radiator fan not working, water pump faulty, coolant flow restricted somewhere, faulty thermostat.
The other, and most serious, reason could be the cylinder head or cylinder head gasket is faulty. When this happens exhaust gas can find its way, under pressure, into the coolant circuit, thus 'boiling' the coolant. A decent garage can perform a "sniff test" to detect the presence of exhaust gas in the coolant.
I think you need to contact the selling dealer to have the car properly inspected/repaired, or look into returning the car for refund. I'm not too genned up on the procedure but have read that the buyer has a lot more rights than they used to have.