I've now flow to Abz 4 times and returned 3 times (the other time I went to Gatwick instead).
At 5.30 am they only had 2 -xray machines working out of 14, but it still only took 10 minutes to get through and into the BA lounge, where they don't have enough information screens (but nice bacon rolls first thing).
Although it is about 5 miles further than T1 (I come from North London), the delays in the tunnel and the drop off ramp for T1 mean the actual time from home to check in is the same.
Getting from domestic arrivals to LHR Express is much better
In summary, it's better than T1 for domestic, but when all's said and done, it's still a terminal