This is why I tried to show a more reasonable example, as opposed to throwing around your entire stable of top-level spells with an oddly multi-classed character. I agree that compared to other top-tier evocation AOE spells vs. multiple targets, I think CME may not turn out too extreme in actual play. But where it completely blows away other spells is in single-target damage with spells that have a ton of attacks.
You are using a 9th level slot with concentration, you need to make numerous attacks and you need to have a way to get those attacks. It does damage and damge only. You should blow away everything else!
However if you take the SR+CME example, that is 495 damage average on round 2 IF
1. you hang on to concentration for 2 rounds
2. The target is within 15 feet and not behind full cover on round 2.
3. Niether spell is counterspelled.
4. No one casts Dispel Magic on you between the 1st and 2nd rounds
5. You burn both your 8th and 9th level spell slots
6. The target takes full damage from Fire and whatever element you choose for CME
If you get all these things right you do a tremendous amount of damage the 2nd round.
Overall there are BIG negatives with upcasting CME IMO. Mitigate those and it can get very high single target damage which is great if that is what you need to win the fight. Don't mitigate those and it is a terrible use of your highest level slot. If you need something other than damage or if your party can accomplish this without using your highest level slot(s), then again is not very powerful at all.
Is it powerful? It can be. It can be the exact right spell that makes a particular encounter trivial by using your 2 highest level slots at 17+ level ..... but then other 8th and 9th level spells will do that too in the right situation.
For example - Take your average legendary Terrasque. This is a relatively common foe for adventures at this level. I have fought them twice in play with a high level party (one time a buffed undead Terrasque) and beat both, the first relatively easily, the second undead one in a rather difficult fight that was started with low resources. Use this technique above on it and on average you do 0 damage to the terrasque and about 80 damage to the caster .... and you are probably doing it from inside the Terrasque since you need to be within 15 feet to use it and he probably swallowed you. Burning a 3rd level Haste to give your best fighter another attack is a far better use of your spells.
To make this discussion a bit more realistic - can we bring some monsters into the White Room? Can you list some of the monsters from official content that you plan to use a 9th level CME against?
Edit: Dangit, I just cannot remember that this spell is self-only!! Somehow that just doesn't make sense in my brain for an AOE spell. Let me update:
Here's an 11th level wizard casting CME as a 6th level spell and then MM as a 5th level spell on a later round:
CME adds +6d8 per attack. MM at 5th does 7 attacks, for +42d8. Total 7d4+7+42d8 = 255.5 avg damage.
To put this in perspective, casting fireball as a 6th level spell is 11d6, or an average 49.5 damage (with a save for half). It's AOE, so ideally you get 2 or 3 targets - we'll say 3 targets for ~150 damage. That's still far less than the ~255 average damage from the 5th level magic missile (that cannot miss) empowered by CME, and which you can distribute damage much more evenly and effectively.
CME does not work with MM because it is not an attack. Also you used both a 6th and a 5th level slot, 2 actions and need to hang on to concentration, so using 1 fireball to compare it to is not really a fair comparison. Two fireballs cast at 5th and 6th level that each hit 3 targets are going to do a combined 220 damage (before save).