If HP is your only concern, just spend it elsewhere.

However, i90 MNK STR is a little bit different than your current MNK's STR (the primary attribute you focus on). If you had T5 and Ex primals on farm, I'd say it wouldn't matter. But since you don't, you probably should be gearing up the class you take into coil.

As someone mentioned, your myth purchases really won't matter here in a month due to the cap possibly being removed and T1-5 being farmable.