No no. IAS on offhand weapon is not calculated when reaching breakpoints.

Three things are taken into account:

• average of the base attack speed of the two weapons

• IAS of the primary weapon

• IAS from other non-weapon gear

According to the IAS Calculator, with a Phase Blade with 30 IAS on it as mainland weapon, and a Colossus Sword as offhand, wit slvl 20 Frenzy, only 10 additional IAS is needed to hit the 5 frames breakpoint. Which means of the Grief rolls 40 IAS you don't actually need any on your gear. For any other roll you can literally put IAS in any slot (Goldwrap, Nosferatu's Coil, 15 IAS jewel, Laying of Hands, etc) and achieve the max breakpoint.

OP, I currently use a Death and Oath on my Frenzy barb. If I had a Grief, I would pair it with Death for the huge crushing blow. Oath is fine as a starter weapon, but it doesn't compare to Death and the 50 IAS on Oath is unneeded with a Grief in a phase blade.