So I purchased the IS-6 and Tetrarch package (as I mentioned, I already own most of the other package and care little for the Valentine or SU-100Y).
One game so far.
Can't really add any observations about the IS-6 that haven't already been made: Gun is terrible, armor is bouncy. (Though I did have to reincarnate my driver at one point.)
Deficiencies overcome by getting so close to targets that weak spots are difficult to miss, sidescraping, and doing an Operation Kelly's Heroes on the second Ferdinand.
The question that I have not yet been able to answer is which regular Soviet light I should train the Tetrarch's crew in (though frankly the T-127 is a better crew trainer), since the MT-25 is a suicidal option and most of the other Soviet lights are just blah. Maybe the BT-7 for low-tier trolling at a tier higher than the Tetrarch.