If a man wears a toupee, must he remove it to put on the head tefilin?
Short answer, yes. See Dvar Shmuel.
The long answer is that the halacha deals differently with different parts of the tefilin- the ketzitza/tatura (the housing) and the retzuos- those retzuos that hold the knot/tefilin as well as the retzuos that don't.
The Rashba learns that in theory, chatziza only applies to the shel yad where it says al yadecha-actually on your hand, whereas by the shel rosh, bein einecha is not a surface, but a guide to the placement. Being a loner in this area, and the fact that no one wore the shel rosh over their hat, he did not want to pasken lema'ase. Based on this Rashba, the Rivash permitted to put tefilin over a hat if the wearer had a serious condition (great distress) since it was better than not putting tefilin.
Even if bein einecha was a reference to putting the tefila actually on your head, that wouldn't refer to the retzuos, only the ketzitza part. Nevertheless, there are those who are machmir for the retzuos that are especially in the place of the kesher and even those retzuos that hold the tefilin and kesher. The retzuos around the arm are not a problem at all. (In fact, I personally do not remove my watch to put on tefilin).
From Rabbi Mansour (7)Same with another point,Yes (Mishna Berura 27:16). If someone is embarrassed to remove his toupee in public, he should wear Tefillin privately without the toupee (Halacha Berura).