No, it is not needed but if you car is racking up the miles, you cat could be clogged. Just switching to a new stock cat would give you more power than a clogged cat. In my opinion a high flow cat is the "icing on the cake" when you already upgrade your header and full exhaust.

And about being worth it, i say it is, because for the amount of money we spend on exhausts doesnt give us tons of power anyway, why not spend 175 on something that would give us a tad more.

EDIT
If you plan to use jdm header or header with 2.5" collecter, yes, you have to get a new cat with a 2.5" collecter.