I had the opposite experience - I was never outparsed as SAM/WAR with Soboro in any party I was in between 50 and the 70s when I started XPing in merit parties with LS mates occasionally. In fairness, I had capped GKT/STR/Crit merits the whole time and top-notch gear barring some luxury items, so I was in better shape than pretty much everyone I XP'd with.
This is my experience as well. However, I found Hagun was at best even with Soboro until 75 - it is heavily gear-dependent for best results. Pre-75 you pretty much have to pick between Haste and 6-hit (and the Haste build isn't stellar anyway), and that does not favor the Hagun's strengths.
I just started leveling my sam at 50 now, and soboro is very awesome sauce.