I like to use Ranged Alacrity and Repair spell power on mine (Hand of Tombs). That bumps the ML on that arm from 9 to 17, but masterful shard drops it to 15. I like using it in Haunted Halls and few other places for the disruption.
Keep in mind rune arms are considered trinkets for crafting purposes. Here it the link to the Cannith Crafting Planner,
http://cannith.cubicleninja.com/
EDIT, BTW most arty spells are Evocation, you can boost your DC and UMD if you craft Evocation Focus II (Passive +2 equipment bonus to DC) as the prefix and Persuasion (Grants a +3 competency bonus to all charisma based skill) as the suffix. ML would be 11.
Also, there are lots of other cool stuff you can craft like dodge, etc. Play around with planner and you can fill some gaps and/or become mo uber.