We've been using an AT&T Homebase since February plus a Mobley for the past month or so, with both subject to deprioritizing at times. While we do sometimes see slow downs during peak usage times in higher density areas, I've never seen our speeds drop below 3-5 Mbps minimum. And then only for fairly short periods of time. Even when we're not subject to deprioritization we sometimes see slower speeds on heavily congested towers, just as we did with our previous costly Verizon 4G/LTE plan.
Keep in mind, the Mobley is subject to "deprioritization", not "throttling", for the balance of the cycle after the full speed 22MB is used. But keep in mind, that's not a full time reduction, just something that can occur during excessively busy periods, with a return to normal speeds when the congestion clears up. I think you'll find that AT&T's towers are not as busy as you might think even in larger cities. And in testing, I haven't had any problems streaming video content using our 1-3 Mbps Verizon 3G service, so that 1258Kbps (1.258 Mbps) may not be as much of a problem as you think it will for the short times it occurs.