Joining a unit doesn't change the IC's unit type so the unit that is Infantry might get the buff but not the IC.
The IC is considered part of the unit but not in all respects.
Either way I don't think it would even be possible :
Master of Deception takes place before deployment.
IC's join units during deployment either to the table or reserve.
So the IC isn't joined to the unit at the time the Infiltrate SR is conferred upon them.
"Infiltrate" says that units that contain at least one model
with this special rule are deployed last,
after all other units (friend and foe)
have been deployed. And, most important an Independent Character without the Infiltrate special rule cannot join a unit
of Infiltrators during deployment
But nobody says "no" to an Infiltrating independent character joining a unit, hence granting the whole unit infiltrate...as long as he is infantry as you, magpie, said.
About "outflank" the thing is this: during deployment, you can
declare that any unit that contains at least one model with this special
rule is ouflanking.
So, after a good check on the rulebook, i must say that my csm in raiders cannot infiltrate.
@mynameisgrax: thanks for the input. in fact I might drop a terminator for a 10 man cultist squad...i'll do the math and modify the list