As i get it. Your initial idea involved hard restrictions on further leveling, while my involved the deformation of the XP curve, so it is not only about Acts/Areas (even though I liked the potato anology). In the former case player can feel being railroaded, while in the latter it would be less noticeable, if at all, because you'd still get this XP if you want to stay in Act 1, but it'd just be a drop of water in the ocean.
Non linear way should pose a non linear challenge, IMO. If you want to rush towards Baldur's Gate as a lvl1 character, you can try, if you are skilled enough to survive the road. Killing one tough enemy in the Moonrise area would grant you 3 levels at once and ease your challenge. This is a reward for going off the beaten path, but this is also a risk to it, that you get potato (s)mashed.
