So if you are replacing with pex, are you running to a manifold or with the home runs? What size pex are you using? It's all going to make a difference in how your tankless performs. 1/2" pex is only going to get about 2.5gpm flow. 3/4" pex could get you about 4.5gpm.
As far as the low flow, it's not just a nozzle on the end of the faucet for kitchen faucets with the pull down nozzle. They have a small diameter hose that is used on the pull down part, and it severely restricts the flow. Nothing you can do about it. It's easy to check. Turn on the faucet full and fill a gallon container and time it to see how many gallons per minute flow you get. The biggest flow will be from the bathtub faucets, those would be 4gpm+. The pull down kitchen faucet you will be lucky to get 1.5-2.0gpm. Which means twice as long to get hot water. Then you need to check the specs of your tankless to see how many gpm of flow it takes to activate the burners and make sure your lowest flow faucet in the house is enough to turn the burners on. The shower heads are the lowest flow usually, unless you take the restrictors off.
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