Zigbee devices can achieve very long range when using more power, but they are not licensed to use that amount of power under the profiles that most home automation devices are designed with.
In the past there have been people who have used external antennas but generally for zigbee pro devices.
However, you may have to get an operating license for some of these devices depending on your jurisdiction.
Beyond that, because the Zigbee devices normally used with smartthings are very low power, range always depends very much on local conditions. Wi-Fi interference from nearby networks, the amount of humidity, architectural features, the amount and type of landscaping, etc. experiments done at any one location are not likely to say much about operations at another location outside of the normal published parameters.
So I’m sure you’ll find some interesting data as far as your own property, but it may not say much about how the same devices would work someplace else. 