I raided only the trash mobs in ToV live (top guild held a monopoly on NToV). I recall the proc happening about once every 7-10sec

a) This is a small sample size. Its not even worth looking at if it's under 1000 samples.
b) This is an unslowed Flurry Drake. NPC spell procs use a straight percentage, unlike players.

I did quite a bit of testing on these and I'm pretty happy with the current rate.

I think it should be based on NPC Dexterity unless that is what you are implying from the straight percentage.

You could tone down the procs/flurry/rampage somewhat on slowable mobs with Cripple/Dex reducers.

Cripple/Dex reducers were a BIG time proc rate reducer.

We would have a team set up just to debuff. It was off the chains on live. That was when you could stack bard tash proc and enchanter tash to get slows to land also.

High Priest is working on bard stacking.

Praise be his name.