The spell says: "A creature in the area of more than one fiery burst is affected only once."

How could you possibly deal 160d6? 

.... this isn't homebrew this is official content.

how to destroy barovia with a single action.

Scribe wizards can change damage types of spells as long as they have a spell of the same level so they can change the damage to something like force damage or lightning because of storm of vengeance or blade of disaster so this spell becomes more powerful for scribe wizards and if you happen to be level 17 wizard / level 3 tempest cleric  storms wrath channel divinity makes any damage that deals thunder or lightning damage  do max damage instead of rolling

Can this be upcast ?

Power word kill is a really bad spell

Clerics should totally get this spell. I wanna go Sodom and Gomorrah on these losers!

If you could aim the meteors the spell would be overpowered, it could deal a lot of damage

Well YEAH! It is a 9th Level Spell. It's meant to be overpowered.

"blazing orbs of fire plummet to the ground" this makes it seem liek thier projectiles, dont u think? meaning that if a 10th level monk used deflect missiles and rolled max and the meteor swarm damage rolled min, the monk... would catch a fudging meteor?

It's not an attack roll, so deflect missiles won't work on them.

absolutely broken spell

It's very very powerful and is a holdover from older editions. But...

The balancing factor is actually the ridiculous size of the area. Bear with me, as a DM, throw a few low level NPCs into the mix, and this spell quickly becomes a liability for the PCs.

The BBEG has a bunch of orphans in cages strategically placed around his lair. Also, the PCs's favorite baby animals. Also, piles of documents that are needed to prove their friend didn't murder the king, or whatever.

Suddenly the precision damage of single target spells becomes more important.

Sure the Evocation wizard can stop 10 of them getting instantly evaporated, but... what of the rest?

The massive collateral damage caused by AoE spells should be an issue for good PCs.

and well it would be a wonderful spell if it actually hit