I think it completely ignores PerlIO in favour of speed.

Nit: sysopen doesn't stop PerlIO from being used. Either your Perl uses PerlIO or it doesn't. (Perl is built to use PerlIO by default since 5.8. I wouldn't be surprised if Perl didn't work without PerlIO anymore.) A given Perl only supports one kind of file handle. It doesn't matter whether you use open or sysopen.

That doesn't mean there is no difference. When using O_TEXT on Windows, Perl might let clib do the LF⇒CRLF translation instead of using the :crlf layer. That could be faster, but I would hope it's not.