in FM Crusader clans CBS still had original model SL era mechs in it ... I would totally upgrade those. Maybe go for the stop gap upgrade and use the Improved weapons from I.O. -- the improved SRMs are totally worth it, same weight, better range ... done. ACs are same as from the original TRO 3050, but that is still some mass savings for HS or more ammo. LRMs iirc got to half mass, but might have min range ... still a win. The improved Gauss rifle is only off by the clan standard by 1 ton iirc. Improved versions of PPC, LL, PLL are ok too.
Upping to full modern Clan standard weapons and armor would just make all these SL models just kick-ass. The IS XL engines might make them a bit more fragile though.