From the Shadowrun FAQ:
QUOTE
When a spirit uses Possession or Inhabitation on a character, are the dual entity's attributes limited by the character's maximum augmented attribute values?
No. Both powers represent a merging (temporary with Possession, permanent with Inhabitation) of the character's physical body with the form of the spirit. For the duration of the possession/inhabitation, the dual entity's maximum augmented attributes are equal to (character's attribute + spirit's Force) x 1.5, rounded down.
Channeling doesn't change this, it just allows the vessel (magician) to act if she wishes to do so.