Hi allz i have amost the same problem like @lpedretti. Using zcashgpuclient_silentarmy build from git for ubuntu 14.04.
rig1   GPU   4   39 ms   0 C°   27.515 sols/s    2 cards
rig2   GPU   4   39 ms   0 C°   27.211 sols/s    2 cards
rig3   GPU   4   39 ms   0 C°   27.713 sols/s    2 cards
rig4   GPU   2   36 ms   0 C°   25.237 sols/s    1 card

I cant understand why the rig with 1 card make almost the same like theese with 2 cards

config rig4
server = "coinsforall.io";
port = "6668";
address ="t1WAhAPgwbk8J3rHAfwbHHAggg2QjuBM7Lt";
name ="rig4";
platform="amd";
onCrash="0";
devices=["1"];
worksizes=["256"];
threads   =["8192"];
corefreq  =["-1"];
memfreq   =["-1"];
powertune =["-1"];
fanspeed =["-1"];

configs rig 1-3
server = "coinsforall.io";
port = "6668";
address ="t1WAhAPgwbk8J3rHAfwbHHAggg2QjuBM7Lt";
name ="rig1";
platform="amd";
onCrash="0";
instances="2";//tryed with and without it no change
devices=["1","1"];
worksizes=["256","256"];
threads   =["8192","8192"];
corefreq  =["-1","-1"];
memfreq   =["-1","-1"];
powertune =["-1","-1"];
fanspeed =["-1","-1"];

Please someone help me, 10x in advance