How low you have to cut depends on how much clearance you need between your steerer expander plug and your top cap in order to compress the whole headset assembly. I dunno what that top cap looks like from the bottom and how far it protrudes into the steerer. But that measurement is critical. Cutting the steerer below the upper clamping bolt is definitely too short, however. Because that bolt is there for a reason and your stem will only clamp to the steerer with the lower bolt.
Personally, I prefer to cut the steerer in line with the stem and have one small spacer below the top cap, just for peace of mind. With one piece bars like this one however, this becomes increasingly impossible due to the shape of the top cap.
Thanks for your very helpful answer! When i posted my question i was not even sure if the top "stem cap" (which is made out of carbon) is even capable of providing enough integrity to compress all the things together.
I attached another image showing the bottom and side view of the top cap. The distance between the two red lines is ~ 5mm. So when i cut the sterrer tube flush with the stem it would give me 5mm room for compression, which is probably ok. I could probably also just cut it 1 or 2 mm above the stem (leaving me with 3 to 4 mm) and it should be fine, right?
What you mention in the second paragraph is exactly what i would have done if i would have gotten a flat stem. Unfortunately i could not find anything for such curved top caps and stems...