I think what your missing is the 2.5litre was designed to flow more air from teh outset. It wasnt teh capacity alone that increased teh power, it was the capacity in conjunction with the hardware that allowed it to flow to its potential. (Manifolds, cams, valves, heads)

Our point, (well, most who has spoken, and i do concur) is that incresing an engines capacity ALONE will not normally improve your BHP reading, it will only allow it to shift its limit of air a little SOONER, hence they normally drop teh power down the rev band and increase torque at the bottom end as a By-product.

As an example, if you took your scoob engine and bored it to 3.0 litres, then put it back together with no changes whatsover, i would predict its low end torque would improve and its peak power stay within 10% of its output at 2.5litres.