I'm glad you have an idea of the statistics of it. I ran some tests and it does seem like the average of fireball is 28 normal, 34 exploding. I'd have expected that to be higher, given the roll being 8d6 and many more 6 rolls being available.

So, I also looked at the max of it though. I ran just 100 fireball rolls, and the max hovers around 50 to 70 for exploding. I ran 100 fireball rolls, 100 times. The max of the 100 rolls on average is 62, and the max out of 10,000 rolls is 82.

What I'm saying is you need to consider that out of 100 fireball casts, you need to expect it at some point to hit pretty damn high even if the average is 34. You change the whole shape of the probability curve here. It's not just going to hit 34 on average, and have the same chance of hitting 60 as 8. 100 fireball casts isn't too crazy, so it might be that one time you pit them against a boss and it deals 70 damage even if it's low probability. Not having a max means unexpectedly, boss fights might end prematurely.

It's a super strong ability I think. I don't think it's impossible to make this work, but having on average 34 instead of 28 for a fireball cast is pretty strong alone. I would maybe balance this out a bit more and maybe do something like each dice that hits its max can only explode once. So you roll the 8d6, you take out the 6's, reroll them and that's that. If you're going for the excitement of it, I think that still makes it exciting even if it's not as powerful.

What I think you mainly want to avoid is that you give this sorcerer this ability, and the party tends to always have them use spells that exploit it just because it's the smartest highest damage move for the sorcerer. Also, you want them to be able to share the spotlight, and having one character's ability be overpowered can make that difficult. I guess that's kind of normal but still, that's the cons I see.