If you can afford running 7-CURRENT, you should really try ZFS.
Even if you don't want to use ZFS file systems, you can still use ZFS
ZVOLs and run UFS on top of them.
For example:

        # zpool create tank mirror da0 da1
        # zfs create -V 10g tank/foo
        # zfs create -V 20g tank/bar
        # diskinfo -v /dev/zvol/tank/{foo,bar}
        /dev/zvol/tank/foo
                512             # sectorsize
                10737418240     # mediasize in bytes (10G)
                20971520        # mediasize in sectors

        /dev/zvol/tank/bar
                512             # sectorsize
                21474836480     # mediasize in bytes (20G)
                41943040        # mediasize in sectors
        # newfs /dev/zvol/tank/foo
        # mount /dev/zvol/tank/foo /foo
        # newfs /dev/zvol/tank/bar
        # mount /dev/zvol/tank/bar /bar

Now, when you want to grow tank/foo:

        # umount /foo
        # zfs set volsize=30g tank/foo
        # growfs /dev/zvol/tank/foo
        # mount /dev/zvol/tank/foo /foo

Not to mention you have fast UFS snapshots, clones and other cool
stuff:)