I think it's a bit of both. Sigurdsson's game is what it is, and he's at his statistical best, when he's right behind the striker. Having him played further back, would completely change his positioning on the pitch, him arriving late into the box, and picking up the ball in dangerous areas would change. He wouldn't score as many goals, or get as many assists. Like it or not, that's what wins games. Which he does for us on a regular basis, so credit where it's due.
Unfortunately for us, he has to play there to make up for Dom's deficiencies in front of goal, and that is a Silva decision.
Im intrigued by the idea of a 433, but can't see the who the midfield 3 would be. We struggle starting passing moves and playing through the middle so I don't think dropping Sig would help in that regard, as he's probably our best passer (in the final third). It's puzzling.
We will need to change something to get better though.