I use the Mk 1 Cockpit Replacament by ASET, Mk 1-2 IVA Replacement by ASET and the Kermantech MK3 Replacement. And Ship Effects Sound Mound, Through the eyes of a Kerbal and Audio Muffler.
The fps drops in the Mk 1-2 were slightly less then in the Mk 1 and Mk 3 Cockpit.
The Muffling Issues were exact the same as the others described above for the Mk 1-2 but didnt appear in the Mk1 Cokcpit and Mk3.
The FPS drops only appear in the in helmet perspective
But not in others like :
or external, etc.
Maybe Ship effects generates a (too) huge Load of data to be processed by Audio Muffler?
Fps drops were exact the same for the Mk1 Plane (seen in the pictures above /30 Parts) as for the Dynawig Stock (110Parts) which i used to test .
So i dont think it has to do with other cpu demanding Mods which i have installed (Kerbal Krash System, Kerbal Launch Failure, Dang It)