Another tipping thread because of the ambiguities of it all...

A lot of this has been said already but my two bits...
Personally, I tip around 15% (give or take) for dim sum, cart or no cart (and mainly because I'm brainwashed to do so). That's probably on the high end for local hole in the walls and average for nicer places. My understanding as well is that the tea fees serve as service charge/gratuity.

Just as a counterpoint, in conversation, I hear that old timers that frequent local hole in the walls barely tip 10% if at all.
For a whole in the wall, I think a good guideline is 10% topped up (plus tea fees). The reason I say this is because of the assumption that the service is generally subpar relatively speaking (ie. cramped table, takes forever to get refills on tea, have to wipe down your bowl & chopsticks, plates take forever to get removed, etc). Note, however, I'm not complaining per se as it's par for the course. Tip more obviously if you have a better experience.

As to owners taking all the gratuities, I've heard of that practice too but it's likely dependent on the restaurant (owner) vs a standard practice. So as a customer, unless you get friendly with the staff, you probably won't know that it's going on. We used to frequent a neighbourhood joint that was eventually sold off to new owners. A handful of staff stayed on and we got the inside scoop. One of which was that tips that were previously pooled & distributed amongst staff were now taken by the owners. Of course, staff eventually started to ditch (and the restaurant I hear is up for sale again).