I totally think he could own the 800 WR and even go under 1:40.
Think about it. Wariner will never get the world record in the 400; he can't even break the American record! But he's run a very respectable 43.45, so imagine if he just slowed it down and raced over 2 laps instead of 1? He could be our first sub-1:40 half-miler -- he just needs to go out in 48 (which, for him, is probably a warm-up for a work-out), then come back with a 51 (which, for him, would be like jogging). That would bring him home at 1:39.
I think this is realistic compared to other runners. That 49.5 pace is about 88% of his maximum 400 speed. Compare that to Wilson Kipketer, who ran a 1:41.11 with a 46.85 best 400 time. Let's assume that if he ran the 400 more frequently and trained specifically for it, he'd run around a 45.5. His 800 pace would be 90% of his best 400 time -- understandable given that Kipketer was more a middle distance runner than a sprinter. You get a similar number for Seb Coe.
As another hypothetical exercise, imagine if Wariner moved up to the 1500. If he slowed things down to a 53 (practically a walk for him, and only 82% of his max 400 speed), he could run a 3:18! I wish he'd give it a try!