I would agree with your analysis. If you have even a chance of using weapons other than your grenade, I would take Furious Attack. It is a noticeable Tier 1 boost and is conditional, but the condition is almost always met.
Just from personal experience, there are times that I have found ETI and Head Count appear to conflict in their resetting of the cooldown timer, or there appears to me to be some circumstances where, when I have both HC and ETI spec’ed, it seems like I get some weird cooldown results from time to time. I am not 100% certain about that, but just my recollection. Something to pay attention to and I thought I would point it out.
You probably still want to take ETI, but if you wanted to make sure you get the most consistent results, spec out of ETI, put 3/3 in HC, use a Reflux to proc cooldowns via HC, and pay attention to whether your cooldown is any better or worse with ETI than it is without it.