There are a few considerations missing in these numbers. Let’s take a look.
#1: weapon damage of various options.
I’m going to give the numbers with 10% damage added on in all cases. This doesn’t change the relative percentages, but it’s truer to the rolls you’ll be looking for. So:
1H Sword (1.4 aps): 2233 avg damage
2H Mighty Weapon (1.1 aps): 3996
2H Mace (1.0 aps): 4314
2H MW is +78.9% vs sword.
2H mace is +93.2% vs sword, and +7.96% vs 2H MW.
#2: AS increases Leapquake damage somewhat, because it reduces the frames required for Seismic Slam.
Based on my own analysis, plus D3 planner’s frame table for SS:
With Furnace, Slam takes 54 frames
With Tribes, 49 frames
With IB, 28 frames
So, a full cycle of 3 Leaps + 1 Slam:
Furnace: 205 Frames
Tribes: 200 Frames
IB: 179 Frames
A GR is 900 seconds long, which is 54,000 frames. In that time, you can execute different numbers of these cycles with each weapon choice:
Furnace: 263 cycles
Tribes: 270 cycles (+2.67% vs Furnace)
IB: 301 cycles (+14.45% vs Furnace)
If you are looking at a smaller increment of time, say, 90 seconds, the number of cycles goes down, but the relative proportions remain the same.
So, IB is actually giving you a non-trivial amount of damage, compared to Furnace, from added EQs, due to higher Slam speed.
#3: Hard-cast EQ is generally applied once every 3 cycles, so:
Furnace: 87 times
Tribes: 90 times
IB: 100 times
#4: An IB setup will have one extra CDR or STR roll, compared to Furnace.
For reasons I’ll get to later on, the ideal rolls on Furnace are Dmg%-AD-CDR. For the moment, let’s assume our IB has the extra CDR roll. That gives us 40% CDR when using Furnace, and 46% when using IB.
#5: effects of extra cdr:
With 40% cdr, Threatening Shout can be performed every 6 seconds, or 150 times per rift. With 46% cdr, TS can be performed every 5.4 seconds, or 166 times per rift.
In addition, WOTB’s cooldown goes from 54 seconds with 40% cdr to 48.6 seconds with 46% cdr.
#6: cumulative effects of added AS and CDR
The damage bonus of WOTB is actually about 1.7x when active (1.5x from Insanity and about 1.13 from added CHC).
With 40% cdr, your actual average buff from WOTB is ((20 * 1.7) + 34) / 54 = 1.259x
With 46% cdr, it’s ((20 * 1.7) +28.6) / 48.6 = 1.288x, an increase of +2.3%.
IB can deploy 903 EQs from Leaping, 100 from HC EQ, and 166 from TS, for 1169 total.
Furnace can deploy 789 from Leaping, 87 from HC EQ, and 150 from TS, for 1026 total.
1169 / 1026 = 1.1394,
1.1394 * 1.023 (WOTB bonus) = 1.1656, or in other words, IB gains +16.56% damage from extra AS and CDR.
#7: effect of extra CHD from 2nd Emerald:
With max rolls on gear, plus BR, you have 57% CHC.
(570 * 5.8) + 430 = 3736
(570 * 7.1) + 430 = 4477
4477 / 3736 = 1.1983, or a 19.83% increase.
#8: overall “base” damage increase:
1.1983 (chd) * 1.1656 (AS + CDR) * 1.3 (IB multiplier) = 1.8158
Still not as much as the 1.932 you get with Furnace. But, we aren’t quite done…
#9: effect of +24% AD
Going from 154% AD to 178% is a 15.58% increase in your AD, which gives an actual damage buff that depends on the number of targets within 10 yards:
1 enemy: +0.0% dmg (1.8158x total)
6 enemies: +9.4% dmg (1.99x total)
11 enemies: +11.8% dmg (2.03x total)
16 enemies: +12.8% dmg (2.05x total)
21 enemies: +13.4% dmg (2.06x total)
26 enemies: +13.8% dmg (2.07x total)
31 enemies: +14.2% dmg (2.07x total)
(This number trends towards the AD % increase, in this case 15.58%. But, 30 adjacent enemies is getting close to the limit of actual density in most cases.)
So, you can see that for just flat-out damage, IB marginally surpasses Furnace when you throw even a small number of mobs together. 1.99 is a 3.2% increase over 1.928, and 2.07 is a 7.4% increase.
#10: Elite damage of Furnace
This one goes pretty handily in the favor of Furnace… +50% elite damage is damn good!
#11: why you need CDR on Furnace
It’s because you don’t have +24 max fury like you do with Blade of the Tribes. This makes it harder to cooldown your Leap via Slam, which cuts into your damage (and also makes it more likely you’ll get hammered into the ground by a berserker, blown up by a molten explosion, or cut in half by arcane beams). You can remedy this by taking Gogok, but this means you can’t take Zei’s, which is by far the best 3rd gem in terms of damage: up to +16% damage (almost 1 GR) even at zero yards of distance, and considerably more at larger distances. Tribes + Zei is considerably better than Furnace without Zei, so you need that extra CDR with Furnace in order to ensure that everything runs smoothly, while being able to take Zei.
#12: conclusion
Furnace will be better than IB… but not by much.
Addendum: Blade of the Tribes
Because it can have that +max fury roll, you don’t need to take CDR when carrying Tribes. Instead you can take STR (along with AD + dmg%) which will generally be best, especially At low paragon, where carrying Tribes can modestly outperform Furnace. Once you pass about 16k STR, however, a primal Furnace with AD-dmg%-cdr will be a little better than any possible Blade of the Tribes (about 4.5% more damage).
Addendum 2: IB armor bonus
The IB armor bonus is not super valuable, since it is additive with the +25% from paragon and the +375% from MOTE4.
The additional 30% from IB adds 5-6% additional mitigation. Not nothing, but not essential.