Admitted Thorn lover here,
Yes this gun is powerful, but its only powerful now after all the other primary weapon classes were adjusted. So how to fix this issue, is rather simple. Undo some of the nerfs. Red Death and Suros Regime both have a powerful healing factor, both could be seen more often in PVP, If the health effect pops after the mark then the mark is removed effectively. Red Death seems fine the way it is but auto rifles need a buff because they were effective in Thorn ranges before they were nerfed, now they can not compete. Return 1/2 of the damage nerf back to auto rifles, would help with this.
In addition to allowing auto rifles to return to importance. I suggest new perks for weapons and armor
CLEANSING- precision kills with this weapon have a chance to remove all curses from the player (about 25% chance)
PURITY- DOTS put on players disappear more quickly (Time of Mark of the Devoured will tick for 5 instead of 7, and weight of darkness will be removed 2x as fast)