If it's the same without a thermostat, then the coolant isn't adequately slowed as it moves through the cooling system. If it isn't adequately slowed, it doesn't spend enough time in the radiator to cool and the whole system essentially heat soaks.

Opening the gasket holes likely makes this worse, as does enlarging the holes in the distribution pipe. You don't want to increase the water flow - you want the water (and coolant) to stay in the radiator long enough to get cooled down.

It's counterintuitive, but this is why a Volvo without a thermostat typically runs hotter than one without.

My guess is that an unaltered gasket and tube, and a good quality thermostat will resolve this.

Cameron
Portland