Interesting that a key feature that the competition already has won’t be shipping at launch. I wonder if this is related to the Homepod’s previous delays.
As I understand it, Sonos can do some of these things because their audio is transmitted over a proprietary wireless connection, so it's not competing with Internet traffic. I use AirPlay today over ordinary home wi-fi, and it's awfully unreliable — stutters all the time, sometimes doesn't play, long pause before audio will start, etc. — and larger buffers would certainly help here.
There’s about a 2-3 second buffer with current AirPlay. It warns you of this when use use the a GarageBand app.
Try enabling IGMP snooping on your router and see if that helps.