What I can see more and more often is, that many render engines actually have problems, when tracing that many alpha planes. I even stumbled over extrem cases where a 2000 poly lowpoly LOD of a tree rendered much slower compared to the original (about 150000 polygons).
The main problem when going the very detailed approach is the amount of memory needed. Of course you can instance this tree without much impact - but then you probably only can load two or three of these "monsters" until you are out of RAM, unless you have 8GB+.
So I really think it would be a nice approach to generate such leaf (and probably even tiny branching) geometry at rendertime would be veeeery nice - but for sure very complex to implement.
In any case, during autumn a hero will be born, I am pretty sure about that.