Implication of polymorphic Amount to round-trip conversions

The current Amount proposal allows code such as

let amt1 = new Amount(2.5, { unit: "meter" });
let amt2 = amt1.withNumber(amt1.toNumber() * 2);

Without the helper functions withNumber and toNumber, the code is still possible, though a bit more verbose, perhaps worth moving into a helper function:

function doubleAmount(amt) {
    let number = Number.parseFloat(amt.toString());
    return new Amount(number * 2, { unit: amt.unit });
}

A concern here is that if developers are encouraged to use such a pattern, the resulting code is less efficient than if they had not converted to an Amount, if the Amount is stored as a decimal-like type.

Start with issue #72 and the current Amount proposal, focusing on the withNumber(), toNumber(), and helper-function examples. Determine whether round-trip conversions should be part of the proposal or avoided for efficiency when Amount uses a decimal-like representation. Done means the conversion concern has a documented design decision.