[Shodo]

Those calculations are not correct. Worn haste does not stack, as stated above, so you would only be getting 31% haste with the mithril 2H regardless of wearing the belt. So your Rod of Lamentation is going to beat mithril 2H by an even larger margin.

Also, while it won't make much difference in this case since the weapons are of similar delay, you shouldn't just use the base ratio for comparison. Your actual modal hit will be 2*(weapon damage) + damage bonus (11 at level 60). Which is why a 1/2 ratio Jade Mace does more dps than a 3/5 ratio Wurmslayer at higher levels for example, even though the latter has a higher ratio:

Jade Mace: (2*9 + 11)/18 ~ 1.61
Wurmslayer: (2*25 + 11)/40 ~ 1.53