XP formula is most likely redone already, since the prices are increased by ^1.35 (which contradicts your 3rd point).
If the current formula is about price of items, then, ok, you're right. Indeed plates and all armors/ weapons in general will cost a lot more, but still, as price will grow up, who will buy plates and plated mails ? Now, I think the plain mails will be the most used armors, combined with leather armors. Therefore, there will be a loss of XP compared to before, when everyone was dressed as a knight, with +3 items and shit... This is why we
need a new formula. It's how I see things, it may be wrong, just feel free to do the calcul with 10k of equipment and see what you can buy, and then, see how many XP it will give.