My characters are in their high 60's expect for Vaan who's in his high 70's. I have to do a few quests and have already completed the game, my HP is around 4700 - 5400 probably, when not equipped with Bubble magick. At level 99 I can imagine having 7000 - 8000 natural HP.

Is it worth it? Yes, depending on whether you have the patience to endure it all, and pick the right locations to level up. In caves or tombs it can be tiring and appear endless, but in other places it can be nifty.