I just saw the announcement of a new Fedora kernel with this change note:
A large change over previous kernels has been made. The 4G:4G memory split patch has been dropped, and Fedora kernels now revert back to the upstream 3G:1G kernel/userspace split.
A bit of googling indicates that the 4G:4G patch is needed for systems with a lot of RAM (eg. 32 GB or more) because the kernel memory tables scale with the size of physical memory and a 32 GB system uses 0.5 GB for the table, half the kernel space available to a 3G:1G system. A 64 GB system won't boot because all of kernel memory is needed for the table.
<http://www.uwsg.iu.edu/hypermail/linux/kernel/0307.1/0246.html>
Was this reversion done for performance or for some other reason? I'd guess most consumer systems won't need 4G:4G as they won't have anything like that much memory, and it only makes sense for an enterprise class server.
