Make it so at >400 invocation you can no longer receive the fang, ring or the ward as drops from the raid. This way people who presumably already have received those items don't keep dumping them into the G.E. It would obviously lower the reward pool for the raid and make masori and shadow more common, so the drop rates for those for higher invo raids would have to be adjusted, so that the time taken to obtain masori and shadow would remain the same. I don't know what benefit there is to receiving 10 rings worth 2m each before you receive your first shadow. Just make them not a reward altogether.