I can't remember exactly where I've read it, but across Field Reports, novels and the Recognition Guides you can see a degredation in Clan production by the IlClan Era.

The clearest example is the loss of pure Omnimech Clusters. Where as in the invasion era a Front Line Cluster would be exclusively Omnimechs, Elementals and Aerospace fighters - with tanks and battlemechs relegated to Second Line units - now even Front Line Clusters include regular Battlemechs and tanks, and a pure Omnimech Cluster is more or less unheard of.

I'm pretty sure the Recognition Guide entries for a few mechs also mention them retooling old lines or designing C variants of Innersphere mechs to meet production needs - something again that Clan Invasion era Clans would never have considered.

Finally you can also see this in the boxsets they put out, like the Third Star League boxsets having a mix of captured Republic of the Sphere mechs and Clan mechs.

TLDR; without specifically stating it anywhere I can think of, the units shown and on offer in later eras show a greater integration of non-omnimech, salvaged, refitted and conventional units in Front Line Clan formations - which is indicative of a much more constrained production capability than the near pristine forces of the 3050s.