Lets make Lightmass EPIC (and understandable)

Hi guys,
I’m trying a pretty easy and fresh new scene.
Dont’ know what’s wrong but I can’t get sharp shadows, it just doesn’t look right.

Light maps 64 - Render time 6 sec :

https://dl.dropboxusercontent.com/u/368267/unreal%20sharp%20shadows/SNAG-0860.jpg

Light maps 1024 - Render time 16 min :

https://dl.dropboxusercontent.com/u/368267/unreal%20sharp%20shadows/SNAG-0862.jpg

Here is the project:
https://drive.google.com/file/d/0B6Tg0tCDvtd8V0xlYzFUbjRSLVk/view?usp=sharing

UE 4.11 Preview 4

Any ideas?