Offhand crits give you an additional deep wound 'roll' of your main hand damage range (probably an unintended legacy feature from the old, useless deep wounds).
This can as much as double the effect of deep wounds if you are using the slowest possible MH and the fastest possible OH (normalization might have some cut offs - daggers probably aren't worth it), I haven't done any tests outside self buffs yet, but this seems powerful enough to boost dps by a very significant amount.
Additionally, Whirlwind offhand crits do not proc deep wounds currently, this bug can result in a pretty sizeable damage loss, especially in multitarget enviroments.
it is only slightly more, like 30dps.
Testing on Dummies will give you different results when it comes to raid buffs, deep wounds damage receives more buffs in a raid enviroment compared to offhand damage.
Deep wounds was changed with the new build, it now behaves differently when you apply a new 'stack' of it by critting, I can't seem to replicate my old numbers quite so easily with the changed mechanics, making it feel slightly more balanced (have seen 2.3k ticks self buffed on dummies in the previous build).
Unfortunately offhand WW crits still won't proc deep wounds.