We use R600 unleashed APs for dedicated wireless requirements during corporate events and have never seen a problem like this before. However saying this, I wouldn't recommend that you ran 400 users over just 3 APs. Although they are within scope, their throughput would be subject to a bottleneck experience.
I assume that they are all connected to the same VLAN and route traffic through a shared gateway? Are the APs connected at 1Gbps from the switch? Have you added a second LAG link per AP? Is there any rate limiting from the switches or APs?
400 is a pipe dream right now i would be happy to get to 200. all aps have a single 1Gbps link to the switch on their own VLAN. COuld you possibly explain more about this 2nd lag link ? this hasn't been brought up by ruckus support yet.
My understanding is the R710 is the only AP that supports LAG or LACP.
What are your channels set to?
Do you have any QoS in place i.e rate limiting in place?
"The backbone is all aps joined to there own 2500 series Aruba POE switch. Directly out a 100Mbit fibre connection." : So you have 3 PoE switches and each AP is connected to a different switch? How are those switches connecting to your core router? Are you daisy chaining them?