I think the IP effect procs when you get hit, not when you attack. So your attack speed may not matter.
Assuming you essentially have 100% CHC (and near-maximum rolls on CHD), Exarian will give about + 5% damage. Sankis is definitely better.
So I guess the question is: is EF (in cube) + Sankis (in hands) as good as IB?
Well, I did a bit of messing around in d3planner, and it looks like getting 5 EF stacks will generally give you a boost of around +50% damage (though of course, you won’t always have 5 stacks). An extra elemental roll gives you +14.29% damage. Combined, that’s about a 71% maximum damage buff, with the uncontrollable effect of periodic doubled toughness from Sankis/IP thrown in.
In comparison, IB’s attack speed buff gives 20-21% extra damage, all of the time, which, combined with the 30% straight damage buff, is about 56% extra damage, plus about 18% more toughness from the extra armor.
The slightly higher base weapon damage of the Sankis setup is offset pretty evenly by the higher base speed of the IB setup.
So, that means that if you could keep 5 EF stacks up for the entire rift, you’d do about 10% more damage with the Sankis setup than you would with the IB setup. That’s obviously not possible, but in a good rift, you’d probably be able to stay at full stacks a fair amount of the time while in density, which is when you would make most of your progression. RGs with adds would theoretically be better since they’d help with EF stacks, but those adds would also eat some of your Stricken stacks. Of course, they’d eat your Stricken stacks if you were using IB, too. Against RGs without adds, you’d be at a significant disadvantage vs the IB setup.
Bottom line, I think the IB setup will be better overall, for most players, most of the time, though not by a huge margin.