The design space of "heuristics soup" is much larger than the space of "clean designs", which gives the "cleanly designed" FAI approach a speed advantage. (This is my guess of why someone might think "cleanly designed" FAI will win the race for AGI. Somebody correct me if there are stronger reasons.)

That certainly seems like a very weak reason. The time taken by most practical optimization techniques depends very little on the size of the search space. I.e. they are much more like a binary search than a random search.