Overpaying for an assets is always a bad decision and it ensures that you will earn suboptimal returns.
Whether you are an investor or a ceo allocating capital, it does not make any difference.
Munger is of course right because he's taking into account the return on incremental capital reinvested.
In your case, I think it's worth noting that:
1) if your company has excess cash for a buyback it means that it does not have enough internal opportunities for reinvestment At 40%
2) shares of such a company will hardly trade at a low price. If bv = 100, 40% roe = 40 in earnings, at 15x this equal a price of 600. A buyback at this level equates to 6.7% yield, way lower than your starting 40%.
3) if a company earning 10% roe is selling at 0.5 bv a buyback would be the best course of action as it would yield 20%.
You can Pay a very high entry price and still get value, it all depends on your assumptions about capital retained and roiic.
There are qualitative aspects to consider of course.
If you think high growth lies ahead don't be too conservative in your assumptions. But this does not mean that you can any price. It must incorporated in your estimate of fair value.
I hope I have expressed myself clearly enough. Hopes it helps, love the discussion.
best,
G