I've just got the Cornish mainline extension for Western Mainlines - and so far impressions are extremely good. The line is beautiful, it has its challenges with the gradients and speed limits especially for heavy freight with long stopping distances.
I think there will be a lot of mileage (pun intended :-)) in also adding the Cornish branch lines. I'd certainly pay for a branch lines add on to this add on, many of the branch lines are set in stunning scenery and have interesting features (e.g. the Looe branch - while we can drive from the china clay works up to Liskeard going under the main line twice right now with the released Cornish main line pack, it would be nice also to do the passenger workings down to Looe as well).
So please consider it