Would it be possible to, instead of 'hiding' or 'culling' the inner leaves (not really sure what the correct term is) instead turn the inner leaf blocks into the non-transparent version?

My thought process is that the trees with culled leaves look very sparse, and this change would just make them keep the sort of thickness that vanilla fancy leaves has without as much of a performance hit.

Is that what you mean:

If yes then try this resouecepack:
Smart-Leaves-1.0-§31.16+§o.zip

Officially dded in Cull Leaves 2.0.0