Well, I'm a bit disappointed. I checked some lens options in the menu (no lens, generic 45°, 45° with the S/N of the lens) and the difference is marginal.
As easily available test objects I measured the max temperature of two light fixtures and the min temperature of a plant.
| | 45° with cal | | 45° w/o cal | | no lens |
Light 1 | | 37,8 | | 37,4 | | 36,7 |
Light 2 | | 37,8 | | 37,5 | | 36,0 |
Plant | | 21,1 | | 21,3 | | 21,4 |
Values in °C, measurements are done with FLIR Tools. The measurements directly on the camera show no difference.
In the EXIF-Data the focal length is changed from 18 mm (no lens) to 9,6mm (with any 45° lens enabled), but in FLIR-Tools "FOL 10 mm" is always displayed.
So you can probably just use the 45° lens without calibration and keep the hack