Obviously Arien is at least 4-C and Tilion is at least 5-C. However, it may be difficult to scale other Maiar to them, as power seems to have little relation to occupation separate from which Vala they served: Tilion was a hunter in Orome's company but was far weaker than Arien, who watered flowers in the gardens of Vana. It seems that each Vala has a "court" of Maiar individually suited to them and their powers, with a few very powerful named spirits and hosts of weaker, nameless Maiar; Manwe and Varda had Eonwe, Ilmare, Olorin and the bird-spirits, Orome had Tilion and other, certainly comparable, Maiar hunters; Aule had Sauron and Saruman, Ulmo had Osse, Uinen and his nameless servants who pulled the freaking sun out of the sky, Yavanna had Melian (who was strong enough to drive away Ungoliant)...
Moon level is starting to be on the low side for named Maiar once we realize that a number of lower-tier Maiar mentioned only once can pull a collective Star level feat. I'm starting to think that the Balrogs driving away a hungry Ungoliant is increasingly less PIS and more representative of how powerful an average Maia really is.
Also, Arien is likely much higher than 4-C, according to this passage from Of The Sun And Moon. "Then (Morgoth) assailed Tilion, sending spirits of shadow against him, and there was strife in Ilmen beneath the paths of the stars; but Tilion was victorious. And Arien Morgoth feared with a great fear, but dared not come nigh her, having indeed no longer the power." Keep in mind that this was relatively early in Morgoth's power dispersal, so his avatar was still incredibly dangerous and capable of withstanding the combined energies of all three Silmarils for thousands of years at a time.
Overall, 6-B seems far too low a baseline for the named Maiar we have, especially the Istari. Manwe treated Olorin with a great amount of respect in their councils, which indicated he was one of the greater spirits as well as wisest, and Cirdan perceived that Olorin did in fact possess the "greatest spirit" out of the Istari. Add to that how the original 6-B scalings for the Istari were based on Osse, who should be at least comparable to Tilion and likely far higher--Tilion, after all, was just one of Orome's hunters while Osse is Ulmo's right-hand man--and it becomes clear that pretty much all of the Maiar pages need to be upgraded significantly.