I bought an TLR RM-2 and have been experimenting with the placement.
Just farting around in my dark basement when I had it on the bottom the barrel really did throw a shadow when you were at low ready, so I moved it to the top. I think having it on top would be OK if you had a fixed front sight base behind it, or a shorter handguard. I have a 10" handguard on order so I moved it to the rear to simulate where it will be.
When you sight in on something far away you do not even notice it, but when the new handguard gets here that will give me a 45 angle option and I plan to try that.