I flew in Y+ on Qantas Dreamliner last night and have to say, not a fan of the built in air bag on the seat belt, it interferes with the tray table sitting flat, even for those who have plenty of spare inches on the lap belt.
Personally I feel that airlines using this type of belt need to:
1. Enable you to slide the tray table an extra inch forward to clear the bulk of the airbag,
2. Adjust the hinge so that the tray table sits an inch higher above the lap to allow better clearance, or
3. Find somewhere else to position the air bag (why not have it built into the seat or wall in front).
The large guy across the aisle from me had to undo his seat belt completely to use his tray, i just loosened mine (probably to a point that was useless safety wise) so i could squish is next to my leg to get a flat tray.
People who dont fit comfortably in Y are more likely to pay to be Y+ or J, but this option goes away because of the poor design of this air bag belt.
Im not petite but with the belt done up i still had a ton of extra belt length available, yet found it uncomfortable, i think less people will leave this sort of belt fastened during meal service or if they using tray table for the laptop - reducing safety over all.