Markel BV per share is around $1530. Compound at 10% for 10 years = $3970. Apply a 1.45x multiple to BV anywhere near the end of that period (not unrealistic, as it was aroudn that level as recently as Jan 2025), and per share price is something like $5750, a respectable 335% return.
Berkshire -- take $350 BV/ B share, grow at 10% for 10 years, apply a (higher) 1.6x multiple, and you get a B share around $1450, 290% return.
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Under these assumptions, you are right that MKL is a potentially a better investment.

Markel might be a meaningfully higher return proposition for those who might be interested in a shorter time frame. Maybe.

Consider this line of reasoning:

There is no particular reason to think that Berkshire's valuation multiples will change much from today's levels, so ongoing returns will be on the order of value growth. I'd probably count on inflation + 7%/year.

Markel is a bit less predictable, but let's say that the on-trend value growth might be roughly 1%/year lower. So let's say inflation + 6%/year. But the valuation appears to be quite low at the moment, presumably in part because of the ~$200m recent hit from bankruptcy of a provider they had a fronting business for (more specifically because the collateral turned out to be insufficient as the claims aged). If history is a guide (which it may or may not be), it's likely they'll be trading back at or above 1.4 times book (versus about 1.13 now) within a couple of years. So the investment could be thought of as "a one time gain of around 25%, plus inflation+6%/year on your money while you wait."

Let's say US inflation is 3.25% in the next few years. Let's say it takes four years to reach the target exit valuation rather than two. On those assumptions it's a 75% nominal return, just over 15%/year compounded, about 5.75%/year higher than the Berkshire expectation with the assumptions above. So, if your investment horizon is partly for a trade like this--sell the next time the valuation is in the 1.4 to 1.5 P/B range--then provided they don't step in another pile of poop before then, the prospective rate of return is quite a lot better than steady-as-she-goes Berkshire. The difference is the greater safety from Berkshire's balance sheet, and a greater unpredictability (though still not that large) of business results.

Modify assumptions to suit, but the idea is that a one-time valuation pop from a good entry goes a long way if you aren't insisting on it being a permanent holding.

Jim