A perk granted on ownership, not requiring wearing the cape: The ability to apply a second instance of the same pouch to a familiar extending the duration by the full period up to 2 times normal duration.
For example, a Pack yak pouch applied to a pack yak with 5 minutes remaining would be extended to 63 minutes while a pack yak with 58 or more minutes remaining would be extended to a maximum of 116 minutes.
In general, I think we should be looking at ways of optionally slowing down, not increasing, the XP rates once level 120 is achieved with a suitable increase in benefits from having achieved that level, trading more returns or functionality for less XP. Once level 120 is achieved what benefit, apart from bragging rights in high scores, is to be achieved from XP after level 120?
There will be some who will want even faster XP above level 120. Others may prefer the just enjoy the rewards while slowing the surge to 200M hence I suggest the perks be optional: a reward or faster XP but no both.