-100res is the cap

Monsters can have negative magic res.
Capped to -100 like other res.

If you swap goldwrap for gheeds, make sure you still reach the 42ias bp (normally obtained from grief 32ias + goldwrap).
This kinda force you to use LoH or HL (which isn’t a problem).

Using dual dwarfs and IK helm, this means your total magic pen should be
-7 gheeds
-10 jewel
Dealing 117% of your dmg
Or 127% with a -10 sunder
127/117=1.0855x dmg increase from the magic sunder

Can 100% ed beat that?
It depends on your total off-weapon ed and str.
No need to include your weapon dmg or any bonus min/max dmg, or crit.
Just look at it this way…
If you have a total of 1170 off-weapon ed% and str, then getting an extra 100% means
1270/1170=1.0855 -> same increase as -10% magic

1300/1200=1.0833 (above that, less effective)
1200/1100=1.0909 (below that, more effective)

All resists in diablo 2 are capped at a range of [-100,100] when applied, unless a specific exception exists like player cold/fire/lightning/poison resistances capped at min(75+maxres,95) and %dr on players of [-100,50] or % dr being capped at [0,100] for crushing blow being applied on everything.
That includes resists that aren't displayed, which are magic resist, poison length resist, stamina damage resist and mana damage resist, all using [-100,100].
And in nm/hell, resists other than magic are set to -40/-100 for players, though magic remains at 0%. So by default, players have 2x the poison length, 2x the stamina loss, 2x the mana loss in hell. And its only -20/-50 in classic for all those

the only really unique thing about magic resist is that it gets no nm/hell penalty. It is actually possible to get a player to -100% magic resist and waste some pierce, but takes the really contrived scenario where that player has a latent black cleft with -65% magic resist equipped and you have the most possible pierce on a warlock with defiler consumed and you can only subtract another -35% resist