Not sure what you mean by "swarm". Do mean cluster? If so, then yes, it's one cluster.
We are not using L3 roaming since it's all one cluster per campus.
At the High Schools where the large clusters are we are averaging about 200+ users on the internal network. About 50-75 on the guest.
CPU usage has seemed surprisingly low. I seen APs with 40+ clients on avergae and the CPU was about 15%.
I really had my doubts, but Instant has really done well so far.
As the device count grows we may start employing VLAN pooling, but as it stands it has work well.