Here's the thing. Clan Tech is only better then IS because for 300 years they didn't have a technological leap backward. The Clans were able to reverse engineer some of the IS weapons specifically the Rotary Autocannons. HOWEVER, their versions weigh more and take more space. The reason being they didn't develop it, theirs is less efficient. Therefor, for the Clans to have other weapons from the IS, it would probably worse, not better.
Weapons such as the Light Gauss Rifles and X-Pulse Lasers were IS attempts at replicating clan tech. A Clan Light Gauss is a regular Clan Gauss Rifle. A Clan X-Pulse Laser is a normal Clan Pulse Laser. So they wouldn't make their own versions, they are inefficient copies of their own tech.
ELRMs, Heavy Gauss, RACs, ect could be reproduced by the clans. But like I said, they would be inferior to the IS equivalents since they are of IS research and development.
Also even though Clan reproduced IS tech is inferior, the BV still gets the Clan tech modifier.. really making them really bad for use in CBT. But if you need a Thor with a Heavy Gauss, thats the only way it could happen.