If your neck is getting sore with a 56 it would likely be worse with a 54 because the headtube would be shorter and bars would be lower. If you are too stretched out you can get a shorter stem to bring you closer in. Also it could just be that it is because you are not used to the road cycling position, and it could also be how you are holding your arms or if you are scrunching your shoulders. If you can, you should take a picture of you on the bike and post it.