I should have elaborated a little more on my original post,sorry for the confusion.
What I meant to say was shako and occy give the extra magic find and they include +5 to skills not add another 5 to skills when exchanged for tals helm and orb.
True you get less life with occy/shako and lose resist.
With the +2 attributes on shako you gain 6 life if you take into account the need for 2 less strength in the overall char build.So thats an overall 82 life loss and 11 mana gain.
The extra +1 skill

+2 tals ammy +2 shako +3 occy =+7 skills: over +3 tals set bonus +2 Tals ammy +1 tals orb = +6 skills) is more beneficial than the +2 lightning mastery bonus on tals orb.
Occy also gives a 20% enhanced defense bonus so the defense loss isnt 140.
The +5 mana /kill also helps fill up the mana orb when killing packs.This was one thing the op was asking about originally.Combine this with magefist 25%mana regen and an extra skill point in warmth gained from shako /occy combo and there is some compensation for giving up insight.Not great but some compensation.
As you pointed out magechick the -15 lr gained from tals orb is a big bonus.However it depends on where the player magic finds as to whether this is really necessary.Tals orb can always be carried as a switch weapon or in stash to deal with troublesome light immunes.I gave myself the option of a sorc carry infinity also:smiley:, mainly if I wanted to run chaos.
I used the build I described with fully synergised lightning,no es,most points added to vitality and merc using infinity.I was mainly targetting meph pindle eldritch shenk thresh with some pubi baal runs and did fine with decent kill speed.
I used anni/storch and charms for resist and my resists were 60-75.My sorc has since retired and I use a barb for mf now ..love horking treasure:laugh:
The main reason I used the tals set was for the mf bonus, the nice resist on the ammy/armour +2 skills from ammy and the 30fcr from the 3 part set.
I guess the shako/occy is a personal choice for the 100%mf and this stems back to using them in 1.09.
Anyway thanks magechick and melianor for jumping on my original post.I kinda rushed the post and should have made it clearer,sorry again for the confusion.