the cost of SELinux is not eliminated by just disabling it at boot time. there is a noticable cost to have it compiled into the kernel, even if it's not used. so it would be good to see a different kernel compiled with all the same options except for selinux
In addition, all the Fedora binaries involve selinux libraries in userspace, and just linking in these libraries can impact performance (there was an interesting discussion a couple weeks ago on the git mailing list about performance issues with one of the tools, and part of the problem was that on some distros with selinux there were many additional libraries being loaded.
unfortunantly testing this with a fully cleaned userspace involves recompiling a lot of the system (potentially including glibc). the only distro that I know of that makes this sort of testing relativly easy is gentoo.