Is arcane still better than infused on jewelry for mag dps?
Absolutely. The only question is Arcane vs. Bloodthirsty. But on the topic of Arcane vs. Infused, Arcane wins hands-down.*
The reason is that there are more boosts to max magicka than there are to spell damage. 20% from being CP300+. 9% or 10% from "good" racial passives. 6% from Undaunted. 7% from Inner Light. 8% from passives available to NBs, sorcs, and Wardens. And 10% in-combat boost from War Horn. In contrast, spell damage gets just 20% from Major Sorcery, 5% from Minor Sorcery, and sorc passives.
What had made spell damage strong in the past was its stronger scaling for light/heavy attacks. But now that light/heavy attacks scale in the same way as abilities, spell damage has lost that edge.
* That said, if you look at the reasons for why Arcane is stronger, you can also see the scenarios where Arcane will be worse. If you're doing no-CP PvP (losing that huge 20% max magicka amp from CP), on an Argonian (only 3% max magicka vs. 9 or 10%) Templar (the class has no max magicka bonuses, but it does have Minor Sorcery for spell damage) without Inner Light (you are in PvP, after all), then Infused will definitely be stronger.