I kind of answered my own question.
So from what I gather from their explanation, Zigbee devices, depending on their specific power and power source, have an uninterrupted line of sight range of 10 to 100 meters. And given that the Iris battery powered devices are on the bottom of the power scale, a 10m uninterrupted line of sight is going to be considered good.
My biggest sensor problems thus far have been the master bathroom (walls, water heater, shower enclosure, metal mesh tile backer etc...) in the line of sight from the hub, and the garage (closest sensor in the garage is just at the 10m mark from the hub, the doors are another 7-8 m away. So no wonder my sensors keep losing communication with my hub.
Contrary to what I was told previously by Iris support, the online documentaiton that I found states that the battery powered devices do NOT act as a range extender for other devices, only the wired / plugged in ZigBee and Z-wave devices.
I am not in a position YET to go swapping in the controls I had planned, so I went ahead and placed a smart plug inside the kitchen, just on the shared wall with the master bath and the garage. So far, no more dropouts. We will see how long this holds...
Worse comes to worse, the v2 Smart Plugs work with Smart Things, so should Iris fail as a platform due to failing to find a buyer, at least I am not out that investment. My V1 Iris contact sensors would likely be junked, but I bought most of them on Ebay, so assuming the hub, keypad, and V1 components are no longer usable, I am still out less than $200.00 on my system and can move ahead with a different rig. I would prefer to stay with Iris as that is where I started, and thus far, like what it offers. But business realities being what they are that may not be an option long term.
Another update for anyone else interested....
My one sensor (Master Bathroom) has started dropping out again as of last night. I have noticed a couple of things.
#1. The drop offs seen to take place MUCH more often during foul weather.
#2. When the drop offs take place, I am noticing network drop outs from my WiFi, particularly the 2.4GHz band.
#3. Scanning from my phone for WiFi, available networks in the 2.4GHz band are all next to nil, and very weak.
I have 2 routers in my house,cabled to each other, the secondary running in Wireless AP mode.
I am wondering if there might be something going on, maybe something one of my neighbors is doing that interferes with the 2.4GHz spectrum. If so how would I find out the what, and where the interference is coming from?