You do not have to run a 48 first 400m to be competitive in the 800m now or ever. If you can run 50 seconds for the first 400m AND slow down by the supposedly ideal 2 seconds, you'd run 1:42 flat, which would have won every championship 800m ever run, except London 2012, and the vast, vast majority of circuit races not won by Wilson Kipketer or Rudisha. If you ran 49.8/51.8, you'd be ahead of Coe's world record. At 49.5/51.5, you'd be within 0.1 s of Rudisha's world record. You'd also be there with 48.5/52.5, so there are many ways of running a very fast 800m. For Isaiah Harris, who does have excellent 400m speed and probably could, with his hs credentials, run sub 46 open with work, the question is whether he can drop from 1:45 or high 1:44 territory down to 1:43s or even better in his career. Penn State training for 800m seemed not to really stint on the speed, with Cas Loxsom, but be lacking a bit on the endurance side. However, I recall from watching some practices online that they would do repeats at a not very fast pace, which suggested that new training stimuli, whether faster medium long intervals or faster speedwork, would help. So, Loxsom probably needed to improve his endurance and could still have worked harder 600s and 1ks to improve strength, while Harris might still work the faster sprint work, harder 400-1k repeats, and improve basic endurance like Symmonds. Finally, form work seems like it would help him more than just about anyone else.