Nixi,

Your 10-manning Kara thread has inspired me (and cost me lots of money), so the least I can do is share some experiences about gearing up my shammies for arena.

First, I'm not sure you'll be able to get the S4 mail helm. The requirement is 1700 arena team and personal rating, something the vast majority of arena goers never see. Personally, I had to pay a duelist to 2v2 with each of my toons to get them up to 1550 for the S4 legs, then 1600 for the S4 chest. Dropped over 1K gold to pay him for the help, but it was well worth it. I'm sitting with S3 helms atm as they have no rating requirements.

As far as hit rating is concerned for arena, Aelli is sitting at a hit rating of 31 and does quite well. My guys are squids, so they have the +1% chance to hit racial. That and my 15 hit rating (from S2 gavels) seem to do fine.

There is no cap on spell penetration, but all the spell penetration in the world does absolutely no good unless your target has resistance to the school of magic you are casting. I haven't worried much about it, but gearing up for PvP normally has given me a spell penetration of 44 (+20 from cloak enchant, +10 from Veteran's Band of Dominance, +14 from Vindicator's Band of Dominance). This is usually sufficient, because most players won't wear resistance gear into the arena. The common resistance buff is MoTW (the druid buff), which gives 33 resistance to all schools of magic. Night elves have a +10 nature resist racial passive as well, but my spell penetration of 44 covers both.

As for trinkets, my guys all have the battlemaster's trinket and my 3 slaves have a +dmg trinket (eye of Quag, might upgrade to Icon of the Silver Cresent). My main has the Medallion of the Alliance so he can trinket out of CCs and for a bit more resilience as he is the usual target.

Hope that helps!