I still think he's best coming in from the left.
In the middle he's swarmed - just like Ozil is - because he is light weight.
Out wide he can be on the periphery from a congestion point of view and use his delivery / fantastic technique.
I wouldn't say Ozil is unsuited to the middle mate, or Gylfi.
But it's very dependent on the set-up.
Ozil has that nimbleness and agility - like David Silva or Mata - to turn out of trouble in a flash. Issue with Ozil is his work rate which Silva and Mata have both improved.
Sigurdsson isn't that type of player. He's not a playmaker who can thread an eye-of-the-needle pass or pick a 50 yard ball like De Bruyne.
He's much more of a sharp touch player in-and-around the area. Gets his assists and goals from quick combination play in the final third or popping up on the edge of the box like he did against Liverpool.
What annoys me the most atm is we had/have a formation/template that works.
As you rightly say, Gylfi coming in from the left is where he's best in that system. He works hard defensively and is always up in the final third (see the build-up to the first goal v Leicester, or Gylfi's goal v H'field as examples).
He dovetails well with Rooney, who in that role as a playmaker can sit on the left of the midfield three with legs around him in Gana and Davies.
When we want to play with two wingers - Bolasie and Walcott - we can easily drop Gylfi back to that Rooney role, but one of them should always start, fitness permitting.