Slitherine

So, I am curious as to what folks think of these two units. I am fairly far along in a campaign and decided to experiment with them at Sevastopol, Rostov, and Stalingrad.

Under the circumstances, I don't find them very good. I think I would rather have an overstrength SFH18 150mm howitzer than either of these two options. Of the two, Gustav is far better on the maps which I have tested them on thus far. It at least has the ability to considerably degrade a long range enemy artillery unit or a deep AAA unit (my campaign is heavily modified so that suppressing/killing AAA is important).

Karl Gerat is only good if there is a target within close range from the beginning (and even then I find the damage underwhelming - it kills like 5/15 entrenched infantry with its one shot). On my Code Blue run through, I managed to fire 3 shots before I took down Rostov. The rest of the mission he was just a liability I had to guard. I expect Karl Gerat to be more useful in Stalingrad, where targets are more densely packed (particularly as I have modified it).

Does my experience bear out with yours?

Would it be crazy to add Counter Battery Fire to Gustav? Or would that be going too far?

Fun but useless. Like you said yourself, there are better alternatives. In the vanilla campaign you can once buy Karl for prestige?! Afair it was like a waste of prestige, so I could need some balancing. Counter Battery Fire sounds nice.

Adding counter battery fire to the Gustav did not appear to work.

Sounds about right, I'm of the same opinion.