Have you seen this video? It's nice because this guy is obviously a good player and he looks at many aspects of the Karavaev whistles, the tuning of the octaves, the responsiveness of high notes, clogging, C natural fingerings, etc. He doesn't hesitate to say when something doesn't seem optimal to him.
https://www.youtube.com/watch?v=8m5FHTyw4ZM&t=14sOne thing to be aware of is that, according to what Karavaev states on his Ebay listings, he uses the same tubing size (22mm ID) for his Low C, Low D, Low Eb, Low E, and Low F.
I know it's common for makers to use their Low D tubing for the adjacent keys Low C and Low Eb, but once you get up to F the bore is getting too big.
For comparison Goldie uses a tad over 18mm ID for his F, and Alba uses around 18.5mm.
Even Burke, who uses larger bores for all their sizes than most, uses around 20mm for his F.
Given that Karavaev uses the same tubing for all his whistles I expect Low C, Low D, and Low Eb to play fine but Low E to feel a tad big and Low F to feel quite big.