I think its common sense that you see pre draft reports match up very well with post GCL reports.... they were both scouting reports from the same season. Odds aren't good that someone got faster, decided to throw harder or all of a sudden improved bat speed in 4 months.
Yes, however -- the competition is much, much better, and supposedly the reports are based on seeing these guys A LOT -- a lot more than scouts see, especially national guys, before the draft. Common sense tells me that even a half season of play in the GCL is going to produce a wealth of new information for evaluators. I just don't think BA gets at that information, nor do I think teams would share much of it for public consumption.