Bodak death gaze alternative

[Voadam]

I'm thinking of changing the death gaze on bodaks from save or die to a death blast that does death effect damage. If I changed it to doing 1d6/HD fort save for half would that be equivalent for CR purposes or too tough as tough creatures now take damage even when they always save?

Normally Bodaks are 9HD CR 8 and the DC is 15.

I'm planning on using a group of eight of them against a group of 4 15th level PCs with great saves and decent hp.