Well, the demo project I uploaded demonstrated/duplicated the issue of diffeerent positioning in groupbox. (It is true that from my original problem, I think there are more issues, but I have not duplicated/repeated these in a way that is useful for this.)
I have not checked your numbers, but 16pixels difference sounds about right. Of course, if one is moving to Lazarus for good, one could just fix this. However, I would like to keep my sourcecode and forms cross platforms. At least for the time being. However, I guess runtime code will be he best option here since I understand if it is too much to demand of Lazarus.