source code is available as tar.xz http://www.kernel.org/pub/linux/kernel/v3.0/linux-3.4.tar.xz
The KERNEL_URI is defined in kernel-2.eclass with .tar.bz2. This enhancement is not hardened-sources specific and if it were implemented, should be done so for all kernel >v3.0. Having said that, I really don't see the big deal. I know its about 15% gain, but is ~10MB in ~75MB really worth the savings?
I don't see the urgency for this change, either.
I came across this bug when looking at something else, but FWIW: commit 99dcf06e308ff01aaf2bcc1890426d0c352a298b Author: Tom Wijsman <tomwij@gentoo.org> Date: Sat Mar 9 21:05:50 2013 +0000 Kernel sources and (gen)patches now use xz instead of bz2. Bug #421721.