Let me think about this for a second. Adding in Boon (via HF Ammy) lets you drop some CDR, which in turn lets you pick up up one of three things:
2k extra STR
16% elite damage
48k LpH
You probably have between 35k and 40k STR, right? So that extra 2k would be 5-6% extra damage.
16% elite damage is 16% elite damage, though I’ve spoken at length on how this stat is not going to be as good as it looks, either in most density or against the RG.
48k LpH is only good insofar as it lets you drop BR: StP and pick up BR: ItF. Before trying that out, it would be worth grabbing a couple spare IBs with LPH, and just seeing if you get enough healing in your target push GR (142, 143, 144 for you) to actually stay alive. If not, this route is no-go. If it works, though, ItF can add up to +40% damage in really good mob types (Horror/Mother, Swarms) when combined with Spear and Stomp, and can reliably get you about +12% damage, even in “just okay” mob types.
That would obviously be worth it, but I think it is kind of a coin toss whether you’ll be able to heal fast enough with LpH alone.
If you can’t use ItF, then there isn’t much sense swapping an excellent FoT for a sub-par Hellfire. Compared to a 20 / 100 / 10 FoT, the HF you’ve got there will cost you about 6% damage.
On a related note:
I forgot a few. Pro-Slam will also add FoT + Nems on follower, as will Lightningquake.
In general, our builds that currently use EW (R6 HOTA, IK HOTA, H90 Frenzy, Pro-Slam, Lightningquake) are going to get a considerably better buff than the ones that use FoT (ZR, VC, Leapquake, LOD HOTA) because adding in a FoT is considerably better than adding in a Hellfire amulet.
I would say that all of the EW builds will gain around +3 GRs of solo power at the top end from the addition of Nems + FoT. For the ones adding Hellfire + Nems, I’d guess +1 GR for VC, Leapquake, and LoD HOTA, while ZR will get somewhere between +2 and +4, depending on whether using BR: ItF works out.