Best neck piece for several STR-modded mono-hit spells such as Vertical Cleave which are seldomly used, but for multi-hit ones like Disseverment there are better options, unless your accuracy is past the cap.
Saying it's best neck for mercy stroke is a little wrong. Math for Mithra with avg Mercy Stroke set (fSTR cap and pDIF static)
STR 67+65 w/o neck
(D+WSC+fSTR)*fTP*pdif
1.base
(41+(67+65)*0,6*0,83+12)*3*2,7=961
2.ire+1
(41+(67+72)*0,6*0,83+12)*3*2,7=989
3.gorget
(41+(67+65)*0,6*0,83+12)*3,1*2,7=993
On triple attack MS you looking at 1634 dmg with gorget vs 1649 dmg with Ire+1 but with less acc (on 2 hits after 1st).
Summing:
1.Single hit goget do more dmg.
2.With Triple attack Ire do less then 1% more dmg with less acc.
3.Adding more str will favour gorget even more.
Also Mandau and MS favour STR more then any other combination (low % of weapon dmg in ws dmg, 60% str mod, 3,0 fTP) there is no WS in game on which ire+1 would be better then gorget on dmg (even counting that 7 str would raise fSTR by 2 points). Sidewinder might me close on dmg (+4-5 base dmg, lower gain on fTP) but you want acc on sidewinder I think.
I remember reading that WS gorgets (for YGK at least), give something similar to +15 STR, so for some WS's (if not all) it won't be that. For multi-hits it's likely you're going to want your sea torque.
Great for BLU spells though, providing you can negate the acc loss (aka sushi/SA)