AirDiver,
This should be fine.
There will be minor changes to the pattern. But as you point out the back plate of the antenna is already metal. And the antenna pattern is directional so the main lobe should be mostly unaffected.
With ANT-17 and also ANT-18 be very very careful to plan for the built-in electrical tilt of the antenna in the E-plane. If you do not factor this in when using this antenna, and use significant mechanical downtilt, you will end up illuminating the area directly under the antenna and nowhere else.
TThe E-plane pattern of the ANT-2x2-D607 can be seen in the attachment. Note that the pattern is aimed at approximately the -25 degree mark. This affects the vertical pattern only; the horizontal or H-plane pattern is symmetric on the axis of the radome.
In the 2nd attachment (building photo), you can see the antenna cluster on a customer building, with one pair of antennas re-aimed properly and the other two pairs aimed incorrectly.