Thank you for the detailed reply. Your findings concur with other comments I have seen on the inter webs (a shame Linn discontinued their forum - shortsighted imho). Linn themselves have stated the upgrade does not yield the same performance as the full fat one - the new clocking, streaming board, and ability for optical ethernet yield the balance of the gains (particularly the clocking). 75-80% of the new one is what most say.
That may be good enough for me as a dCS Rossini replaced my KDS/3 two years ago in the 552/500 system and the Katalyst moved into the other room where it performs headphone only duties. I was hesitant to do the upgrade as at the time it seemed like one last “money grab” by Linn due to the fact they announced it was going end of life. I was reticent spending more money on something officially discontinued.
While I understand the upgraded version is 75% of the new fat version - how much better do you think the upgrade is over the current KDS/3 Katalyst - does it become a different animal?