H Peacock,
If I may add just one comment to what you have said. You cannot really have a meaningful gate height for a barrel roll because the minimum height required at the apex for safe completion is a function of the pitch attitude used in the second half. Personally, in a barrel roll I always look for being wings level when I reach the inverted straight and level attitude (which will be nose slightly above the horizon) at the top of a barrel roll and height only becomes an issue in maintaining separation from any cloudbase above. I do tend to fly relatively shallow barrel rolls with a low apex height because I always feel that steep nose attitudes in the second half from a high apex height are very uncomfortable!