I remember that back in 2002 or so Mikey built two pairs of TFA 2004 Jr's with the Cobalt slices(10 or 12) centered in the coil and surrounded by M4; and he reported that they gave 80% of the inductance of an all Cobalt stack for a considerable reduction of the cost of the all-Cobalt TFA 2004 Jr.
I was fortunate to purchase the 10 slice pair and Paul Birkeland built them into a Custom BH 2A3 stereo amp for me with Paul Joppa's trick dual and shunt-regulated power supply with MQ chokes, etc. The amp still impresses me every time I listen, especially with the JJ 2A3-40 output tubes. There are all-Cobalt B7's in the matching Linestage as well. Hope this contributes to the core material considerations here.
Cheers, Hank in Eugene