The answer to your question is not as simple as it looks.
When the smart guys at NCR designed the footprint of what eventually became the 802.11 standard back in the early 90's, roaming was a feature high on the wish list. It was all but perfectly integrated back then, but 25 years of fine tuning resulted in quite seamless transitions.
Assuming your wireless environment is well set up; AP's are nicely overlapping and have different channels assigned to them there will be indeed some disruption in the data stream when re-associating between different Access Points. But these transitions are low level and chances are next to none that your application will suffer from these transitions.
Once your laptop decides to re-associate,
- It issues one or more disassociation frames to the current AP to
initiate the re-association process.
- It broadcasts a probe request for alternative AP's to pick up.
- It decides which AP has the best conditions to connect to.
- The new AP will issue a probe response, authentication and association frames will be exchanged
All of this actions generally take place within 50ms or so and will happen more frequently when signal levels are considered 'poor'.
You asked whether you'll get packet loss or brief period of re-connection gaps. Technically the answer is yes, but as said before, it won't necessarily interrupt 'human' service.
As long as you're not using general QoS sensitive considered applications you won't notice. A voip call might give the odd extra delay once in a while and streaming video might freeze up for a couple frames. Basically the same stuff that might happen when the wireless network is overloaded or when you have the odd collision on your wired network. But on human being level you won't see disruptions in your webbrowser, FTP download, etc. You won't get kicked off your Netflix or Skype service.
All good news? Not quite. You're talking about an Airport Extreme featuring AC technology, and Airport Express'es which currently only have N-technology and are not available with AC. While the roaming on both technologies are subjected to the 802.11 standard there is no difference. But on speed there most definitely is.
Additionally AC equipped devices have in general much better antennas which might give other issues. Also the AC equipped Macbooks have better antennas. In a traditional setup, Express'es are wireless extenders, getting their network wireless and repeat (sic). Placing them in an ideal overlapping position will be quite a task. And even then the question is whether your Macbook will actually prefer N at x dB or the AC at x+y dB.
As you see the more extreme your setup is, the more your chances are to end up in a vacuum. If you're indeed at - or outside - the edge of the AC/N coverage areas, maybe you should opt for using additional Extreme's instead of Express'es.