If you are getting enough rain you might be overwatering. I haven't watered any trees or bushes in months. Or grass, for that matter. But we've had a very wet summer here. I only water established stuff like shrubs and trees when there hasn't been any rain in a long time and they look droopy. I do water the veggies and flowers a little more often and of course potted plants but that's it. If you are getting enough rain weekly I'd shut it off. As for the cypress, I don't know if they are similar to Juniper but if they are they don't like to be too wet.

You don't say how big the property is, so it's impossible to say if the system is "overkill." Nine zones might be exactly what the original landscape designer intended or it might be a few too many, no way to tell without pictures, etc.

As far as the Master Gardener's comments, mature trees usually don't need anything other than rain, unless you encounter drought conditions. Shrubs, especially non-native ones, will always need some additional water except during the rainy season. Your lawn (ugh!) will ALWAYS need irrigation. That's just one of the reasons to eliminate as much grass as possible.

Italian Cypress do not need the extra irrigation and it may be the reason you are seeing some decline. Turn off that zone or cap the heads that are hitting them and see how they react over a period of a couple of months.

The first question is are you paying for your water or do you have a well? If you are watering all your zones it is very costly to over water. You can also cut the time down in areas that don't need extra water or change some heads to just misters that need less. I have a well connected to my irrigation system which has saved me a lot of money.

The property is just over 1/4 acre. Not big. Luckily I do have a well. After shutting off most of the zones, the Plumbagos immediately started declining (pulling them out anyway, eventually). Haven't noticed any change in the cypress, but I assume it will take a while.

What needs to be watered and how much will vary greatly over time. Generally speaking, a sprinkler system put in when the property is new won't be ideal even a few years afterwards. A lot of water is needed to get trees and bushes and the lawn established, but afterwards less is clearly needed.

Two things that you haven't mentioned: what type of grass do you have and what kind of soil/area does your yard have?

I live on a sand hill and it's almost impossible to overwater anything. I am way above the water level. Where are you? What type of soil do you have? Clearly if you have Bahia grass you will need to water much less than if you have St. Augustine.

I think what your master gardener said to you makes sense. Still, there are a lot of variable and nothing is better than simply observing over time what is happening in your yard.