It's not really CPU intensive. To turn a 5-bit char into an 8-bit one, just do a lookup into a tiny constant array. Doesn't even introduce any new branches. Roughly the same performance as iterating over every character. (which you're probably doing anyway if you need the conversion)

The savings aren't just memory, it's probably a performance improvement too. Fewer allocations, fewer pointer indirections, and some operations (like equality checking) become O(1) instead of O(n) for these tiny strings.