If the S&P takes a hit and returns to a more modest valuation, so will Berkshire.

Sure, in terms of a price dip.

But one shouldn't make the assumption that Berkshire's equity portfolio is exactly as richly valued as the broad market is. It usually isn't.
Price dips which don't start from overvaluation are transient.

When looking at Berkshire's stock portfolio, it naturally breaks down into Apple and the rest.
For Apple, I personally value the business based on the trend of its look-through earnings, not the market value. Seen that way, the current market valuation level of Apple stock doesn't matter to the value of a share of Berkshire.

For the rest of the stocks, last time I checked it looked like the weighted average earnings yield of Berkshire's portfolio was about 7.17%, equating to a P/E of about 13.9.
It's just one possible yardstick, but that certainly doesn't seem dangerously stretched.
The simple average earnings yield among the S&P 500 stocks equates to a P/E of 17.6, superficially 26% more expensive than Berkshire's portfolio ex-Apple.
On a cap weight basis it's 21.4, superficially 53% more expensive.

Jim