A multipoint water healer is connected using flexible hose. This has 1/2" in the connector only but very tiny inner hose (less than 1/4"). What would happen if you use real 1/2" or even 3/4" inner diameter pipes all throughout? Has anyone tried it? How is the flow rate and water pressure?
That would depend a great deal on what the internal passages of the particular device are.
If they are tightly restricted to control water flow and permit adequate contact time for heating, less restrictive piping to and fro will make almost no difference. You can assume this is likely the case if the sort of hoses used are either supplied with the heater or specified by the maker of the heater, which might be the first thing you'll want to check.
If they are not particularly restricted, water would flow faster/have less pressure drop, but as a result either the heater will consume more energy as it will be heating more water, or the water will not get as hot if flow is faster than what the heater can fully heat with faster flow.