Buffett's best years were the Buffett Partnerships, from 1956 to 1969 (iirc), where his returns averaged 40% per year.

Berkshire has benefitted from insurance float for a long time, but it's a minor benefit. Float can't be invested in the stock market, it's extremely restricted in it's usage, and it's only been helpful because of the incredibly disciplined way Berkshire runs it's insurance companies. Proof is it never gave Berkshire Hathaway remotely the returns the Partnerships achieved.

That's not proof at all. 60 years ago markets were much less efficient than they are today, and Buffett Partnerships was incredibly small compared to the size Berkshire is now.

Buffett has said it ad nauseum- the anchor around his neck is the size. Beating the market with what $200 billion to invest? That's leagues different from beating it with $10 million.

That's an interesting argument because basically any modern Berkshire letter you read is going to say something along the lines of insurance being "the most important" part of BRK, an "all-important" component of their revenue, "the cornerstone of Berkshire", "our core business and the propellant of our growth since we purchased National Indemnity", "our main business", "a huge winner", "exceeds its book value by more than any other Berkshire business", &c.

(Can you tell I just plugged a bunch of random years plus "Berkshire letter" into Google? I didn't find a single one that didn't play up insurance; I'm not leaving any out.)

In 1994, he even laments exiting a position in GEICO in the 1950s as one of his major mistakes!