Hey Charles,
If you want to deploy a large number of machines, I suggest you deploy a small configuration (maybe 3 controllers 1 compute) and then run the overcloud deploy command again with 2 computes, so on and so forth until you reach your full allocation
Realistically you can probably do a stride of 5 computes each time, experiment with it a bit, as you get up to the full allocation of nodes you might run into a race condition bug with assigning computes to nodes and need to pin nodes (pinning is adding as an ironic property that overcloud-novacompute-0 goes here, 1 here, so on and so forth).
As for actually solving the deployment issues at scale (instead of this horrible hack) I'm looking into adding some robustness at the ironic or tripleo level to these operations. It sounds like you're running more into node assignment issues rather than pxe issues though.