spl with PAX_SIZE_OVERFLOW would previously trip an overflow warning (see https://bugs.gentoo.org/show_bug.cgi?id=546488). As I understand it, this prompted the addition of "!PAX_SIZE_OVERFLOW" to CONFIG_CHECK in the ebuild. When the bug was fixed, "!PAX_SIZE_OVERFLOW" was again removed from the ebuild (see https://gitweb.gentoo.org/repo/gentoo.git/commit/sys-kernel/spl/spl-0.6.5.3-r1.ebuild?id=5f7e82db697844802307daae159dbdff6740b6cb). However, ebuilds for later versions of spl (>=0.6.5.6) have added this restriction again, apparently without any explanation or reason (afaict searching google and bugs.gentoo.org). Since I can find no reason for this, I assume some kind of mistake was made when writing the later ebuilds and that "!PAX_SIZE_OVERFLOW" should be removed from CONFIG_CHECK in the ebuild.
We removed sys-kernel/spl some time ago.