Hello,
I have two of these guitars and a real les paul(traditional plus). I have checked everything and have the same problem as you. I have cut the nuts very low, adjusted truss rod, adjusted the bridge raised the stoptail etc. I even lowered the action of the reallly stiff one and found it still to be stiff. Lowered the action below factory specs. I think I hit 3/64 on the low end and 2/64 on the high end. It's been a while since I messed with it. I also measured neck angle of the real and the 2 copys. The two copys actually have different neck angles, but are very close. The one that is stiff has a neck angle that perfectly matches the real. It hits right below the base of the bridge above the adjustment screws... I would say about 1/4" below the string. The other copy hits less than 1/4" below the string, but feels much better. So there is more neck angle in the copy that feels better. Keep in mind that neck angle is a control of how low you can get the action without buzz occuring at the upper fretz verses the lower frets. Truss rod adjustments to even out tension caused by the force of the strings. Of the two copy one is heavier than the other. The heavier one is the stiff one. Now keep in mind real gibsons have their nuts cut real high and I have not adjusted it, but it actually plays better than the stiff one with the nut cut high. The other copy that is not stiff plays comparable to the real one with the high nut. With all this said I don't have answer for you, but it may lead you the answer. It could be nuance of the neck angle, but I highly doubt it, because I was able to get my actions down and below factory spec without buzz and still feel the tension. Another possible theory I have is the break angle at the nut possibly could cause high tension. I did change my tuners so the angle has changed there a little bit. Anyways hope this helps.