Webb has never run quicker that 49 point out of the blocks. The 47 you are talking about was in a relay, and I think it was 47.8, so that's about 48.4-5 out of blocks.
48.5
48.5 + 4 (52.5)= 1:45.0 (there or there abouts already)
52.5 + 4 (56.5)= 3:31.87 or 3:47.31 mile (almost there)
56.5 + 4 (60.5) = 7:33.75 (about the same as his 2 mile pr)
So, I'd venture to say that he is already on the 4 sec rule and to improve on his current pb's he will need to improve his 400m speed, or invent the 3 second rule. It's a same we never really saw ElG have a hit out at an 800 at his prime as it would have given us a good idea of his true speed. It's all to easy to speculate on what he would have run. You would think that if Cram could run 1:42.88, the ElG would be there or thereabouts as well, vbut not any quicker.
JR