If you own the new Aurora Contraption only, the XP you earn will be a whopping 2.5 times your previous level. If you buy the Hoodie (which only works from levels 55-), the XP you earn will be a whopping 2.2 times your previous level. If you have both of them, your XP will be 3 times your level without either of them.
People who already have the hoodie who then buy the contraption will see their XP increase from 2.2 times the normal level to 3 times (not a huge difference).
However, to make sure the item has value for people who already owned the hoodie, we've just increased this to a massive 4x normal XP, so everyone will notice a big difference when buying the contraption.
So, to clarify, buying both items will now allow you to level up 4 times a quickly as you can without either of them!