Hi pinspanner, welcome to mightyram. The question about the Mikuni carb is valid. It is designed to deliver fuel below 5,000 rpm and provide a balance of fuel efficiency and torque. The water choke assemblies are like all things - does a good job when it's working correctly. Far as I can tell they also chose the carb based on the air flow demand of the engine which is about 170 CFM (which is 'accurate' for the 2 litre) and having a vacuum actuated secondary allowed for some flexibility in midrange to top end power. But it is lousy for performance as it strangles airflow. If it's running hot and cutting out check your fuel float level as it might be too low. The stock carb should not starve out under any circumstances.

You could've inadvertently created a coolant leak in the base of the carb when you pulled it off so check the mounting gasket and bolt tension. It would be able to pull coolant into the inlet manifold which will cause it to run rough as well. Start there.
As for other possible coolant leaks, they can be in the usual places like a dud head gasket, radiator/heater core or split hoses. But they also pop up in places you wouldn't normally look, like the base plate of the inlet manifold and the coolant circulation pipe from the water pump around the back of the head. They are notorious for getting corroded out. Good luck and let us know how you go.