I'll try and keep this as simple as possible
On a "normal" setup (single LNB) you use the most optimal part of the dish in theory. Manufacturers claim upwards of 70% efficiency.
Now on a multi LNB setup the LNB's are offset so I assume you use less than the optimal focus? and the more off centre you go the more gain you loose?

My question is, by how much? e.g. is using a 1m dish with an LNB offset the equivalent of using an 80cm with the LNB central?