Implication of polymorphic Amount to round-trip conversions

[sffc]

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.

[ljharb]

I'm confused, which pattern are you saying is less efficient?

[sffc]

Versus code that, for example, stores the number as a Number

let amt1 = { number: 2.5, unit: "meter" }
let amt2 = { number: amt1.number * 2, unit: amt1.unit }