return MSI to default 20
how many polygons do have portals?
normals and edges position do not matter, but they need to have as low polycount as possible
I can see you have some strange highlights in the corners, if it's not intentionally caused by round corners, then you should probably add some thickness to your walls (for example shell modifier).
and check it http://forum.corona-renderer.com/index.php/topic,1662.0.html