If a death curse for the average wizard is like a 30, while the heart exploding curse in SF was a 35, what the heck was the level of power Eb used when he killed a couple hundred people with a single motion, and didn't appear even slightly fatigued....
Speaking purely in game mechanics? Probably not that much, actually. Something on the fly like that would be evocation, which uses much lower numbers than thaumaturgy, usually.
The average goon in a fight only really takes maybe 3 or 4 shifts to put down (roughly a solid blast with a shotgun or sword).
35 shifts is super overkill. That number is meant to take into account anything and everything the target might do to try and survive -- 20 shifts of consequences, 5 shifts for the highest defense stat, 4 shifts for the best possible roll, 4 shifts for all the stress boxes and another shift or two to put it all over the top.
The RPG source book puts Ebenezer's magical "Spirit" element attack at a base 8 power and 9 for targeting -- then admits it's probably low-balling him.
Damage in DFRPG is based on the fate system, which uses stress boxes and not HP. Any single hit that "lands" past the stress track is a Take Out, unless the character uses consequences to reduce that hit so it lands on the track.
The max a mortal has before getting powers is 4 stress boxes, so the average goon is going to have 2 or 3. A hit's damage is
(Attack Roll + Weapon Rating) - (Defense Roll + Armor Rating)
So, for instance, assuming average (+0) rolls, Murphy (Guns at 5) firing her pistol (Weapon:2) at a low-level goon (Athletics at 2, with 2 stress boxes) without armor becomes
(5+2)-(2+0) = 5 shifts of damage = one dead goon
Said goon could take a Moderate consequence (reduces hit by 4 shifts) and survive the hit to keep fighting, but generally nameless goons don't get the option because they're a speed bump, not a roadblock.
I'd say in a "high level game" like Changes would be, the average goon probably has a defense stat of 3, so even at the best (+4) defense roll, Ebenezer is still doing more than enough damage for a solid kill.
Now, AOE attacks are done by taking 2 of those shifts of power and using them to make it affect a whole zone, reducing the Weapon rating. Ebenezer can swing that easily and still be packing enough heat to whack the whole lot of them.
All that said, if I were GMing that session, lord knows I wouldn't be rolling 200 times for attacks. They'd probably be statted up as a single, large "monster" instead. Or an environmental hazard.
So, in answer to the question... He probably put more power into it than Harry's average attack spell, but not nearly as much as the heart-explodey spell because it was a bunch of nameless goons.