Prot,
You asked how big my games were to be able to afford 3 groups of 6 deathmarks, the answer is 1850 points. The rest of my list includes two Doom Scythes, two Night Scythes with 7 Gauss Immortals in each, two squads of 6 tesla immortals, overlord with orb and sempiternal weave, royal court of 5 crypteks, 4 with Eldritch lances and 1 with a Veil, and three individual heavy destroyers.
I start with the overlord joined to the royal court and the three heavy destroyers on the table, in hiding, hoping to get second turn. Because they are ever living the possibility of wiping out the overlord and the crypteks in two turns is remote. Everything else is now coming in through reserves.
Overall, I think this is pretty balanced and I have been pleased with the way it works. I feel the other Elite troops really don't cut it and are both twice the points cost of deathmarks. I also know that I am not taking good advantage of our fast attack slots which may well be our most versatile, but the heavy destroyers with preferred enemy make really good snipers, if you lose one (except for first blood) it is not a big deal and it uses up an organisation slot which allows me to field the reserves I have described. So far I have been lucky in not losing deathmarks in deep striking accidents and when my reserves come on, you can rather dictate where the conflict is going to be.