Always believed that 28,000 was the standing Kop's capacity at its peak.
That's correct. It was reduced a tad at the start of the 1978 season when fences were installed at the front and the support pillars were widened at the bottom to stop people shimmying up into the roof.
It was not a steep terrace - the gradient being considerably less than the Holte End or the Gelderd End. Having stood at one time on both those terraces, as well as the Stretford End, I always thought that one of the reasons for the high capacity of the Kop was the narrowness of the steps themselves (as well as the absence of any vertical gangways). Elsewhere the steps of the terrace were wider and therefore I guess there were fewer of them per acre, as it were. That's also why, even when Old Trafford was full, you never got the same sense of being in a 'packed' crowd as you did on the Kop.
Looking forward, it would be nice to see the corners filled, though having read these threads I see why that might not be possible from an engineering point of view. Part of the Kop's legendary atmosphere was to do with engineering I always thought. It simply had great acoustics and that must have been partly due to the fact that there was nowhere for the noise to escape to. The roof came down very low over the goal (remember how 'letterbox' the view was from right at the back of the Kop?) and the sides were also filled in as the terrace curved around to meet the Kemlyn and Main Stands. It was almost like being inside a building. Again, unlike the Holte or the Stretford End where you were more open to the elements.