A language like Pike is filled with stuff that is more aimed to make things simple rather than optimally fast. That's the whole point with it. This is just another such instance, and so the added complexity of a flag should be put in relation with the actual benefit it'll do in this case.
Just measuring conversion in some other case doesn't tell anything. You could just as well compare
gauge (a = b);
with
gauge (a = utf8_to_string (string_to_utf8 (b)));
and arrive at the conclusion that the conversion is practically infinitely more expensive.
Now, the added complexity of a flag isn't very much, so the speed gain doesn't have to be very much either. But it should at least be clearly measurable, I think. Otherwise people will see the flag, think that it'll have a worthwhile effect and maybe start troubling themselves with attempts to make use of it, and in reality they won't gain anything for the effort. That's why I asked for actual measurements.