Few points that may or may not be relevant:

* Avalon 10x slices allow sandwiching between 2 radiators. PSU components are separated, reasonable design feature and this might be favorable layout considering overclocking and extending.

* There might be chain member number limit dictated by serial speed and driving capability, so it might be reason for 10 chip per chain.
Reasonable would be to calculate theorethical max datarate needed to be in the ballpark.