Importing mesh causes dark patchy shadows in game.

So I've been trying to fix this for about an hour now, I might be missing something obvious but I can't find any solution online.

Basically, this lamp I made has very odd, blocky shadows in game:


I've tried all the usual suspects - redoing my edge splitting, replacing my spec map with a blank one, recalculating normals, etc. But it stays the same. 
Eventually I figured out that it's probably the mesh importing process that's ruining it? The shadow mesh once imported into s4s has the same odd shadows:



Whereas it's fine in blender:


Is there any way to fix/stop this? I've tried triangulating + re edge splitting the mesh beforehand but it doesn't fix it.

Here's the files:
.package (ignore the missing LODs haha)
.blend

Thank you for any help! c: