Martin, you are one of the best there is at sleeving, and I doubt the other three people would take credit for your work. =)

Personally, I think it detracts from your work as art. I understand you have a business and can emphasize with the frustration plagiarist cause, but eventually they will be found out.

At the very least I would ask you to consider moving the mark to the side, so that if a plagiarist crops it out they crop out the detail of the sleeve.

Anyways, I think your work is incredible art, and it should be treated and respected as such. Feel free to call out the thieves too, that never hurts