If I may suggest something, I would advise against purchasing Herkumi and Dark Matter. According to
Rawr, this is what the second best trinket looks like:
(the power of the trinkets is in a descending order)
Since you mentioned having reached the hit and exp caps while sitting at 20-25% Arp, I decided to make a custom build for a NE feral cat. If you have Needle Encrusted Scorpion already, stick with it. Although I also agree with Blkunxpd that Mjolnir's stronger than NES, the actual dps gain according to Rawr is like 11, which is why imho you can stick with your current trinket. Don't waste EoFs on Herkumi because with those emblems you can buy something better - 4pt10 piece or mby a new cape or a belt.
As for regemming to Arp, I would advise you to reach around 450 first. According to
EJ, the threshold is 400, but I advise ppl to start regemming at around 450 in case you replace a weak item with built-in Arp with another w/o that stat.