A quote from AWKM about this topic when someone brought up the HF sights elsewhere:
The heavy flak is one of the oldest guns. It was finished before any gameplay feedback got added to it. If we were to use the sights on it, the cannon would occlude a huge amount of screen real estate. Furthermore, the sights don't help for drop. The LJ is a good example of sights that work quite well.
To be quite honest I don't see why that would be a problem and judging from the visible part of the iron sights the Heavy Flak has, they would be quite useful, it might not have numbers, but the multiple circles it has can be used to estimate shots.
Also, Twin Heavy Carronade, that's an example of a gun that takes up a lot of the screen.