Doc, this a good question, however considering pre workout supps are generally only taken once per day - the level of caffeine from them (although high) would be negligable compared to people who drink 10 - 20 cups of coffee per day.
The mechanism you mention here may occur, however I imagine it would play out in such a minor way that it wouldn't be of consequence.
With a U.S pre workout supp I'd be more concerned about anything hormonal in there having an effect on gyno more so than caffeine. I haven't been keeping up with them so mucn lately so am not sure if there is much which has a hormonal effect in any these days.