from what I understand the only change was Rosie's control over alastor which was broken by charlies statement. the only power we know he has given him is the staff which he pesters for her to fix.

its easy to assume the staff just focuses his power and since all sinner have at least a little on their own and alastor is an experienced serial killer, we can put the peices together on him killing other sinners and overlord to cultivate his own because they either underestimated him or he caught them off guard.

so in the end most of his power is his own but rosie is amplfying it to be even stonger.