From my investigation last year, expanding most filesystems and LVM groups is easy. And supported in code. The Linux RAID stuff on the other hand had big red flashing warnings around all the expansion stuff saying not to use it except to test the code.

LVM is also better at handling hardware RAID expansion then a raw filesystem or software RAID layered on top. I run an array at work on a fibre attached Smart Array with LVM. I can expand it, let the hardware finish it, then get around to expanding the LVM and filesystem with little to no user interruption.