I am just about to start playoffs and I have called up three rookies because they are big upgrades offensively over the guys I had playing those positions during the regular season and they are nearly as good defensively as well. Does anyone have evidence to suggest they will perform as well as their ratings suggest they should? Or evidence to the contrary? I do not have much experience with rookies in the playoffs and want to know if I am right to think they will be better than the veterans because of their ratings or if they will fall short because they are inexperienced.

Players perform to their ratings.  Age or experience has no impact.

I've had guys make their ML debuts in the post-season with no problems.

I've done this quite a few times with good results. These were mostly young AAA guys with outstanding ratings who I kept in the minors so that they could get a full season's worth of ABs or IPs rather than ride the pine as backups or relievers in the ML, but who were better than my ML bench players. A key pinch hit in the late innings or a couple of frames of shutdown relief at a crucial moment makes it more than worthwhile, and if their ratings are good enough they are capable of that. Ratings are ratings; I don't believe there is some hidden "experience factor".

When I made the World Series last season I called up my best prospect, a 19 year old 2B whose defensive ratings hadn't quite progressed far enough for him to start yet, but who projects to be a bona fide first ballot Hall of Famer one day.  He hit the go ahead RBI single as a pinch hitter in the 8th inning in his first big league AB.  Also was used as a defensive replacement in LF and pinch runner (has 100 speed).  I got swept but he played in 3 games and did fine.  As tec said, players perform based on their ratings, experience has nothing to do with it.