Ilambris is only good if you're actually doing lightning damage aswell as fire. mDK normally uses only fire damage, thus Ilambris isn't BiS. Skoria aswell is far from BiS due to it's low-esque proc chance and lack of damage compared to Grothdarr. I'm seeing fantastic results in both AoE and ST with Grothdarr, so I'm personally gonna use it for both and putting my Skoria in my bank to collect dust.
Even with it's recent buff, 3pc WP + 3pc Infal is better than 2pc Skoria + 4pc Infal/Moon. Grothdarr, however, is netting well over 3,5k DPS ST and that number only increases when there's 1-2 adds every now and then such as in vMoL.