I think the Polish medium tank line is a great addition to the game. Not a needed one no, but we are in the second half of 2020 and we still got a brand new line of tanks that is BALANCED and is not yet another example of the endless powercreep WG shovels up our asses (I'm still very salty over how the Italian mediums powercrept all other autoloaders into obsolescence, when I see a post-nerf Progetto 65 played by a 45% average get 4.5k damage I cannot help but scream inside).
That being said, there is one thing I do not understand – why make the gas turbine gimmick unique to the CS-63 at Tier 10?
I don't know how "real" these tanks are, and I suppose the whole line is mostly fake, another case of WG taking liberties and interpreting "secret blueprints" or whatever – maybe somewhere in the Polish archives there were projects of a tank with a gas turbine engine, or something.
However, I think that to give some flavor to the line they could have made the turbo mode available from Tier 8 onwards (including the upcoming premium), also to help players familiarize with it as they progress to the top.
I believe this would've not made these tanks OP or problematic, as unlike the EBRs the CS-63 pays for improved engine power and speed with abysmal aim time and gun handling – you either run or shoot, not both at the same time.