Theres few t0 pieces that are absolutely pain to get. Hunter t0 shoulders from wyrmthalak never seen them drop and huge pain to get (last boss lbrs)
Warlock shoulders again huge pain. First you need to beg your team to do Jandice, very annoying boss room never drops either.
Then priest shoulders from solakar, no one wants to do it again annoying side encounter.
But worst gotta be magister boots from strat. So you gotta persuade everyone to do this big hassle to open all the mailboxes to get postmaster and then not have them drop 
no, nothing will boost or improve
and two specs is even debatable
classic about painfull grind, welcome
Edit: mage boots are actually from heartsinger not postmaster but still rare spawn. Some classes have it easier to get full set. All you need to do is look boots and shoulders t0 from Atlasloot those are the ones that differs from class to class. Some classes have their drop on a boss that everyone does anyways. Like shaman shoulders gyth ubrs. Others get short end of the stick like wlock which drop from jandice
there is better gear than t0 for all classes that drop from dungeons