Hi,
I'm using a Metal Groove AC52 happily.
I have both a 6db and 12db gain antenna available.
I'm having to go about 800 ft to get a -70db signal with the 6db antenna.
The vertical alignment is pretty good between the antennas.
How much improvement in signal could I expect with the 12db gain antenna if any?
Both are omnidirectional.
Cheers!

In theory the difference between 6dBi antenna and 12dBi antenna is +6dB. In practice it can be anything between +6dB and possibly -10dB if antennae are not properly re-aligned if needed.

With omni-directional antenna the only way of getting larger gain is to reduce vertical beam width - when talking about 6dB difference that means vertical beam-width of higher gain antenna is ¼ of vertical beam-width of lower gain antenna.

By some definition main beam is where radiated power is equal or up to 3dB lower than its maximum radiated power. Typical omni antennae have their maximum in horizontal direction and as direction changes from horizontal, radiated power starts to decrease. Radiated power of 6dBi omni antennae decreases to half (-3dB) of maximum around 10 degrees above or below horizontal (making it a 20 degree vertical beam width antenna) and 12dBi antennae have around ±2.5 degree beam (a 5 degree vertical beam width antenna).

If the height difference of both antennae of 800 ft link is more than around 20 ft, then the 12 dBi antennae should be mounted tilted (not vertical) so that the other link end enters the main lobe.