What I've always been told and my impressions somewhat support is that clan tech is generally lighter and hits harder, but it also produces more heat and is a bit more fragile. IS PPC vs Clan ErPPC for example. Clan mechs seem to be able to take roughly the same amount of punishment as their IS counterparts, but they can load more gear due to more efficient design. I think the reason the IS kept getting their asses handed to them was the clan ruthlessness and superior weaponry. ALL clan weapons as I understand it outdistance their IS counterparts by a good margin.