Back before December of 2003 Sneak attack used to work as long as you were out of the monsters line of sight. Since the line of sight is cone shaped, you were actually able to SATA onto the tank who had hate if positioned correctly, it was amazing. Here is that fateful update :

"Sneak Attack" will now only be successful when performed from directly behind an enemy.
The positioning for "Trick Attack" remains unchanged.

Give it the @%$# back!

There was also a patch a long while later where they messed up. They made it so SA and TA, stacked or separate, would work on every hit in the weaponskill. It was like that for a couple of hours before they fixed it, so I actually got to play around with it, it was glorious. Make it a trait for high level THF. Tone it down for each successive hit if it's too "game breaking".The point of THF was to plant hate, and you need damage to do that.

Add another ability that works like sneak attack and works as long as you're out of line of sight (like SA used to). Allow it to be stackable with both SA+TA. Just something to boost damage.

Add a job trait that lets trick attack usable anywhere. Basically, being able to use SATA solo. Same damage, just THF takes all the hate from it.

The thing that cripples THF damage more than anything else is other players. Mostly BLMs. Nobody ever really wants to help the THF do damage. In fact I'm sure some players are actively against it. Positioning should be a bit more lax if anything for high level THFs as a job trait.