I know nothing about 8x10 photography, but from what I have seen the cameras are quite large. Is it a possibility the leverage and not weight had a bearing on the performance of the giotto heads you tried deniz?
I know one of my P&Ts struggles with my praktica and its' 28-200mm semi-macro lens, not because it is heavy but because it is long and adds additional leverage to the load on the tripod head.