I think Celtic are likely to be stronger after the January window. That said, if Celtic are ahead in the league, I wouldn't put it past the Celtic board to gamble like they did last season and keep what they have.
I doubt even the Celtic board will allow last season's events to happen again given that they were seven points clear at one point only to throw it away. I'd expect Celtic to get another couple of players in at least to strengthen the squad especially if they have a decent Europa run.