I've played as IS and Clan in CW and it does seem that the IS LL quirks are a bit OP, and that does make them very good for defence. I assume its part of some kind of grand plan to make Clans more close range and IS long range, but it doesn't feel quite right yet.
I think the point is to make it so that when the game gets steam launched people would buy IS mech instead of flocking to clan. Remember that all those clan nerf and we are getting highly customizarble IIC coming.
But all IS hard laser vomits get nerfed. And no more high quirks like DRG-1NN or WVR-6K. And IS MPL get the same heat penalty like ML so no more 7+ MPL alphas without ghost heat.