Worth Tharja?

Try for Quick Sparrow 2 or L&D 3 to get rid of that speed bane.

Well, I ran my Tharja (+HP/-SPD) for quite some time and used her as my main sweeper until I pulled a +ATK Reinhardt. After that Tharja was still of some use, as she was able to cover for Reinhardt's weaknesses (Mainly as counter for Hector). Anyhow, Tharja with Darting Blow is still able to double a lot of the opponent's units especially with the use of buffs. Even though I'm not using her as one of my main units anymore, she's still a reliable force to be reckoned with and worth investing the effort in. Thus, currently she's frequently kicking Veronica's ass in the Tempest Trials.

I've been running a gimp Tharja (+Def, -Spd) since I drew her and she works really well with Darting Blow.

I realize it's not ideal by any means, but she still manages to double a fair amount of units (especially if I launch her off of Eirika) and she's surprisingly useful with Vantage given this spread.

The thing with IVs that's worth remembering is that while there are definitely better setups for certain units, you can still make *most* character setups work if you're creative and work with the understanding that the majority of people you'll come up against probably won't have ideal IV units either.

Also, Tharja's utility in the current tempest trials cannot be overstated. She is clutch to my runs!

In the lower levels of the arena sure. But don't invest too much into her (skills requiring the sacrifice of a 5 star for example) unless you really don't see yourself ever pulling another good mage because she is outclassed by Linde and Nino, the latter of which can be pulled at 3 stars so you have more opportunities for IV lotteries.