I'm pretty sure that 300 is the top ilvl that "matters" for retainers, unless I missed an update. Putting them in better gear will get you further in ShB, yes, but for now it's useless.

And yes, they'll only bring back HW gear - but they'll also bring back stuff like nidhogg scales & jet black dye which can still be hundreds of thousands of gil if not millions, and I'm pretty sure I get the "rare" comment for those, which means it wants the higher ilvl.