120-160dmg from off hand? not that much (amano and masa99 are "only" 132)
+ offhand hit on shoha wont get attack boost

if weapon are no longer treated as 2H MG DW sam have low attack (0 attack trait, loss of hasso, loss of /war) no native multihit (lol zanshin without hasso) and can no longer cap DR, loose aftermatch/relic proc on off hand (if RME sam)
= it wont increase you melee DPS
I realize your lost in the "must prove someone wrong on the internet" but you kinda missed the entire discussion.

We were talking about giving the 2H DW ability either to SAM natively or WAR / DRK. Thus your premise of SAM/NIN is immediately wrong. It would be SAM/WAR or SAM/DRK in all situations and thus an attack boosting JA will be present.

The off hand hit gets the same WSC bonus as the main hand hit and is only missing the fTP multiplier. For a WS like Shoha it would definitely add a chunk of damage, woudln't double it or anything stupid but would be noticeable. Finially your melee damage would be increased via the direct DPS buff that is DW. 30% DW is a 42.85% increase in melee damage, you'd have to be rocking 65~70% haste before the 10% JA from hasso would beat the 30% boost from DW.

Now quick come up with more nonsensical conclusions based on faulty premises then declare yourself the "winnar!!".