Good questions. As I ponder this, it makes me wonder "how sealed does a cabinet have to be before we consider it sealed". Is a gradual pressure loss over 30 seconds considered "sealed enough" in traditional speaker design.
I don't think I've ever seen any data on this, but the primary purpose of sealing is to basically isolate the back wave from the front wave to prevent SPL cancellation. And, these waves are happening in milliseconds. So, it probably doesn't have to be a perfect seal for a "sealed" cabinet to perform near it's optimum.
Having said all that, for my HB, I do "seal" the bottom of my tubes with double sided tape and mylar from an old windsurfing sail (mylar is a plastic and mine is about the thickness of a transparency slide that was used on overhead projectors many years ago). However, the tops of my tubes that are against the plywood aren't sealed so I'm sure there's some leakage there.
Am I getting burping also...probably so during those hard transients. Can I feel the burping or does it make a difference. I'm not sure, but it happens so quickly and so violently that my perception of any effects due to 'leakage' may be getting masked by the overall experience and my system restores to equilibrium within about 30 seconds or so.
But, your question is a good one....if I would perfectly seal my setup, would the transient response be affected for those 30 seconds when equilibrium was upset after a hard transient. And, would my perfectly sealed HB actually hinder the TR capability as you originally were asking about due to the static offset of the driver that happens by just sitting down on top of the platform.
It could probably be measured easy enough by testing the extreme boundaries. I'm thinking of a 1/4" hole purposely made in the HB cabinet (like a hole in the plywood top of my cavity) with the ability of closing and opening that hole. Then demo a few favorite bass scenes that you know caused burping in the past. One demo with the hole wide open, and another demo with the hole closed and see if a difference can be felt and measured using your Qvibe before and after that hard transient event.
Having said that, I'm thinking some leakage may be desirable to avoid the static driver offset that does happen when first sitting on top of the platform.
Just some random thoughts...