I don't know what this even means. I'd already be running v8 runtimes. They're unavoidable. But there's no need to fork and exec (many times over) just to sweep a filesystem tree for files matching patterns.
It's the shell, by the way, that cares about "spaces in filenames". Not dicking around with shell metacharacters is just one of the many wins you get by not shelling out to accomplish basic programming tasks.
I get what you're saying for the "already in a REPL" thing, but as far as fork/exec overhead, xargs does a single fork/exec for every thousand or so files you search; the overhead is tiny compared to everything else.
Not even that, but xargs can also be made to run N processes in parallel ('xargs -P N'), making a find|xargs sed pipe perform much better than anything you could easily invent on your own.
Note that 'xargs -P' behaves very similar to 'make -j' and is often useful as a simple work-unit scheduler.
by the way: "-print0 | xars -0" is really idiomatic, you know? Lucky you didn't have a single filename with spaces to handle...