I would like to know if replacing treacle with molasses in my fruit cakes will change the texture of the cake and how much molasses can I use in a 2.5 kg. of baked cake?
In my experience, any "liquid sugar" (warning: colloquial, not very accurate term) can be substituted well without texture problems. Mollasses, treacle, sugarbeet syrup, liquid glucose, golden syrup, and even agave syrup all seem to work well as substitutes. I suppose corn syrup or sorghum will work well too, but haven't tried them.
You get differences in taste, but in most recipes, the taste of the substitute is a good fit. Most of these tastes rely on caramelization anyway, so you should try to use something of the same darkness because roughly, two "liquid sugars" of the same darkness have the same taste intensity. Using a lighter substitute is mostly OK too, although you will lose a little bit of aroma. Using a dark substitute in a recipe which calls for a light one can result in the taste of the sweetener overwhelming all other tastes. For example, pliable ganache is made by adding liquid glucose to the ganache; I would never use sugar cane syrup as a substitute there.