How far do you think this could work to pull in some 850/860 mhz?
Im roughly 80-100 km's from the nearest metro area with a system in that range. Do you think it would work if pointed in the right direction about 25ft off the ground?
Depends on many different variables. It isn't just about how good your receiving antenna is. If there isn't a strong enough signal in the first place, no amount of gain is going to make it work.
Radio systems are designed to cover a specific area with a certain amount of signal strength. Being outside that designed footprint makes it a real crapshoot. Receiving the signal is only part of the issue. Not getting interference from another location (maybe in the opposite direction) on the same frequency is the other. Directional antennas like this can certainly help, but 800MHz is -really- line of site. If there is topographical obstructions (hills, etc) in the way, heavy foliage, etc, you will have a very hard time receiving it, if at all.
The other issue can be how good your set up is. While this is a good antenna, you can lose a lot of signal in cheap coax, bad connections, etc.
You can try some of the on line propagation mapping tools, but it will be difficult to fill in all the blanks on how the transmitters are set up. When it comes down to it, you may just have to set everything up and give it a try.