Yes, hip/torso separation is important for velocity. But velocity comes from doing a lot of things right. Is separation something you should directly teach (and how would you teach it?) or is it something that can come naturally from doing other things right mechanically? I don't know this answer and I'm not sure if there is a definitive answer that covers all pitchers.
From my experience, I haven't seen much velocity improvement from pitchers who focus on drills that isolate a specific mechanical factor they have arbitrarily picked out. Velocity comes from doing a lot of things right and focusing on just one aspect can often lead to your mechanics getting out of whack and you developing another separate weakness. I would recommend trying drills where you are merely focusing on using your entire body to throw the ball hard. I think numerous different med ball throws and overweight baseball throws can be great for this.
I think there are certain mechanical flaws that can, and should, be addressed by direct mental cues such as maintaining a strong front side. But something such as hip/torso separation shouldn't be focused on. There are other drills that can work on learning how to get your whole body involved in generating velocity without focusing on just one aspect of the separation at the expense of all of the other necessary ingredients.
Again, this is just my view and you could get more comments from others if you can expound upon your question more. Ideally, you would get up some video of a few 100% intensity throws from a mound from both a side and front view.