If the regrowth trips the global cooldown then no, you will be stuck in caster for 1.5 seconds. Not having 2 piece Tier 5 myself, I can't positively answer the question though.

I'm desperately looking for help on how to DPS better.

My guild has 2 feral druids. My druid "senior" and I have similar stats. I use 4t6, while he's using 2t4 + 2t6. He already got hit capped, and I'm very very close that it's neglegible. For some reason, I could never seem to beat him in straight DPS fights. The most interesting thing is... somehow he manage to do more number of white damage hits than me. I would stand there behind the boss just like him, but, for example, he'd land 100 hits while I'd land 80 hits total (no Heroism). He haste no haste rating in his gear whatsoever.

I'm sure both of us use the same standard DPS cycle (4 pts "minimum" combo Rip => Mangle => Shred). The only thing that differ is the fact that I habitually use FFF every time I don't have energy to do anything (while he does a few FFF per boss fight). However, that shouldn't affect anything should it? After 2.3, I also powershift quite often when I have less than 10 energy. But I thought 2.3 instant shift shouldn't interfere with steam of white hits. Please correct me if I'm mistaken. Anyway, my white is usually 36%, Shred 27%, Rip 23% of all damage if it matters.

Do you think I'm doing anything wrong here? Frequent FFF + instant shift shouldn't delay white hits right? Is there anything I can do or experiment on to improve my DPS even furthur?

A regrowth with the 2T5 bonus causes a GCD so you would be stuck in caster form until the GCD is over.

Is he using something like [Drums of Battle] and in a different party?

Purely by virtue of what's available to me at the moment, I'm still using 2T4 in my main tank set (Head and Chest), and whilst tanking anything significant, the extra Rage is superfluous. On trash, it's really nice, and also on threat intensive encounters like Void Reaver. But it's not something I would hang on to if I had options.

As you said, 4T4 is easily quantifiable, and 3T5 is a nice inflection point to lose it (even though you don't get anything worthwhile from 2T5, the extra stats and higher base armor on T5 will cover the loss of 4T4).

I wouldn't agree that tier bonuses don't bring much to the table for tanking, I think 2T6 is quite powerful for threat generation, although T5 bonuses are uniformly crap.

Just throwing out some ideas:

Powershifting resets the energy tick timer, I'd assume it also resets the swing timer (since you're going from a 1.0s swing to whatever your melee weapon is and back.) Does anyone know for sure if this is the case?

Every point of hit makes a different till you reach the hit cap. Not a 20% difference if you're <1% from the cap, but it's still going to factor in. How about expertise? Dodges still count as misses in WWS iirc.

Any instant-cast spells shouldn't affect the swing timer, but have you run tests without using FFF to see if it makes a difference?

Stupid question .. i experimented on a macro today:

and it does exactly that even faster than you can see. Now the question is - while tanking a boss - will using that macro make it possible to be hit while in caster form?

Also this might be an absolute great option to finally use the 2pc T5 bonus

I know how spells and the GCD works and shifting isn't part of that mechanic anymore as far as I am aware.

Perhaps I misunderstood what you wrote. In any case I do recall that the instant regrowth from 2 piece T5 causes a GCD making it a rather dangerous choice while tanking since you end up waiting the 1.5 seconds in caster form to be able to shift back.

Edit: Disregard post, GCDs do not interfere with autoattacks.

There is also expertise.