So sending him through the wall means she's very strong indeed. Just how powerful is a Jhi, anyway?

Well first keep in mind the quality of the wall was never mentioned. Now granted he would have had some means to protect himself but he didn't even seem at all phased by being thrown through the wall. That said, it was still an impressive feat, just not as powerful as some. Also keep in mind that's as powerful as she gets. So as things are now, at her strongest she's only at the Wall power scaling.

As for how powerful Jhi are, there are 4 kinds of magic. Vit which is normal magic, Rooqa which is memory magic (they have perfect memories and can even share memories using their Rooqa if they have one), Bol which is spatial magic (as was seen in the Mistress' domain) and God magic (which draws strength from the entire race, but only so long as they do not speak). 99.99% of the population can only use Vit. Now, given enough time and preparation Vit can do just about anything for as long as you maintain the spell. It can even create life and bring back the dead, but only lasts as long as the person can maintain it. But trying to cast on the fly is so much trickier because we're talking very complex forces to manage and will usually just pick one they like and just learn to do that one quickly or do really easy spells. Think of it like doing complex math. Given enough time you could probably solve just about any equation you wanted. But if you only had a couple of seconds, your ability to do so is extremely limited.

There are some born without any ability to use Vit and these either become Rooqayla if they're one of the seven races that the bone stick responds to, or potentially Bol if they can find a god to give them a Bol staff. There is only one god per race at any time so they're the only ones that use God magic, which is the strongest of all if they keep from talking long enough. A Jhi, then, is someone born who can do all of the other three. There is nothing that can use only 2. You have access to either one, or all three. The reason Jhi are so powerful is because they have access to the perfect memory magic. Whereas everyone else would have to effectively start each spell over from scratch each time, a Jhi would be able to simply remember the final result and produce that effect from there. Rooqa Jhi (those Jhi that also have a Rooqa) are even better at this since they have much more experience with memory magic.

But Bo is slightly stronger at Vit than Alliquinn. But as she is a Rooqayla Jhi, once properly trained, she'd be able to just instantly cast any spell she ever learned, meaning she could cast complex spells Bo couldn't block or complex shields that no one could ever break. That is the effectiveness of a Jhi. It's not that they're stronger than normal, just that what would take a normal person an hour or two to cast would only take them a minute or even just a few seconds.

Ah, so she's more skilled than powerful, then. But why did Bo say "the Akilraen Jhi"? This seems to imply that, just like gods, there is only one per race at a time.

Well, eventually. Right now she hasn't been trained in Vit at all. As for that second part, good eye, but won't explain it because spoilers. Just note that normally Jhi are rare, but you do see quite a few of them, especially in higher ranks of whatever field they go into (Most think of magic as offensive for wartime and that's where most people in world think about them, but you would still see them a lot in academia and hospitals, anywhere where really rapid magic response is extremely important)