I actually looked at Soul Capacitor just the other day
on my blog. The TL;DR of it is that it's not the best consistent damage trinket, but it can lead to the overall highest parses if it procs a lot, and specifically if you can get Touch of Death during one of the procs.
I think Blademaster is much more interesting. It scales off your own stats, and fistweavers generally have a ton of AP, so it should work out well. I've had some difficulty finding out exactly how it's implemented - the pets seem to have some sort of base weapon damage but I have no idea how to find it. The other interesting thing about Blademaster is that the proc seems not to differ depending on which ilvl of the trinket you have, but possibly that goes into the hidden weapon damage. I got a mythic one this week and had a normal so I'll do some testing there, and I'm going to try it out on farm next week and see how it goes.
Especially for super-short fights you have on farm it's probably worth swapping out IG for one of those, and possibly swapping out IRP for the other.
As far as fistweaving on Gorefiend, we've been working on that fight for a while now too, also 5healing. I've built a sort of hybrid gearset with leech/EST (would be IG if I had it) but non-spirit jewelry (I've also tried out 2p/2p but not sure if it's better or not), and spending p1 in serpent but p2 in crane. It contributes a fair amount of dps, usually 4.5-5m per feast, so you're knocking off somewhere between 3-4% of the boss's HP over 3 feasts. With 5 healers you certainly don't need to be in serpent during feast, so I think this is better even if the boss should die in 3 feasts anyway.
I have a question about only 3healing it though - with the healer Shadow only targeting healers, wouldn't somebody get killed on the 2nd wave each p1, since one person would get re-targeted?