Hmm, I have to admit it has been a while since I have had a look at AT Blizzer's setups. I was sure that 'Ist' DF was the end-game weapon as far as efficiency goes. Diminishing returns with MF etc. Otherwise it would be 6xIst weapon.
Some theorycraft:
Bosses HP (2x those of normal monsters) in AT
Embalmed 7894-14732 @33 CR
Horror mage 4530-8786 @0
Invader 10888-19356 @0
Plague Bearer 8288-15786 @0
Standard sorc:
Shako (Ist)
3x Tal (armor ist, amulet, belt)
Fathom (Ist), let's say 23%cd as average
Spirit Shield
40 MF Chancies
30MF Nagels (could be blues with >30MF)
50MF Travelers
cube + Gheed + 33x SC (maximum 231 MF, let's say we get 210).
Overall: 193MF Tal's, 255 MF rest of equipment, 250MF inventory, total: 698 MF (my sorc has 720)
Blizzard damage (not casting BC): 5190-5476, aka 5333 average
This means you would 1 shot Embalmed ~40% of time, 1 shot Horror mage always, 1 shot Invader never, 2 shot him always and 1 shot Plague Bearer ~32% of time.
Removing Ist Fathom for 6x Ist sword means you also have to make up 10% fcr to have a breakpoint, so you need 10 fcr ring hopefully with 15 MF. Overall loss: 23% csd and 3 skills, overall gain 135 MF getting to ~835 total. Effectively that increased your magicfind efficiency by ~2.82%.
Blizzard damage now with casting BC: 3740-3964 aka 3852 average.
This means you can 1 shot Embalmed, but extremely rare, 2 shot happens most of the time, 3 shot is extremely rare. You don't have guaranteed 1 shot on Horror mages anymore, but must ice blast him too. Invader is now 2 and 3 shot material as is Plague Bearer. You should not lose over ~2.82% killing speed because then Ist Fathom wins easily over extra MF from a sword. This means if your sorc runs were 45 seconds with Fathom, now you must be able to do them in 46.27 seconds. So, you should not lose more than a second in runtimes for a sword to be worth it. From experience and numbers presented in here, you will lose damn much more than a second by that change.
As for best AT runners out there, they usually do AT in ~34 seconds at most, and in that runtime you shouldn't lose over 0.9 seconds for MF sword to win. Impossible.
So, yeah, Ist Fathom is end game and unbeatable option because it makes you more efficient not per kill, but per time invested since you will kill considerably faster.
Now doing math for 105 fcr sorcs makes no sense since they are quite similar to 65 fcr variant, but with like 100 MF less. Same story applies for them too.
Now I'm not exactly sure about Pindle himself, there difference could be less since you kill only one target. On Pindle of much higher value is to have harmony bow in town and you can then kill with CtA. I think that is possibly the best variant of Pindlerunner sorc. Or you can use fathom and play without BO, but that seems a bit fragile to me.