Lagat has simply never showed anything to suggest he possessed the monstrous aerobic talent of elite marathoners. I believe the 10k to be the farthest event that prime Lagat could be competitive in on the world stage.
As for the 800, you seem to be under the misconception that the new "super spikes" actually have a significant impact on racing times in general. There is zero statistical evidence of this. There's some evidence to suggest a significant impact on distances 5k and up, but nothing conclusive, and certainly nothing showing that there is a significant impact on 800 times. If anything, they simply help you maintain greater consistency in your races.
That said, Lagat ran a final 800 of 1:46.9 during the Athens Olympic 1500, so I think he could've run a 1:44 low in his prime, and if he were trained specifically for the 800 perhaps a 1:43 mid