the.racking.monkey,
A few pieces of information:
Normally the MTU should only be changed if it absolutely does not work. The MTU should not work most of the time and then suddenly stop. A successful MTU should just work, once negotiated or set manually. It should NOT be changed at all after that.
Here is what the "no broadcast" knob on the wired ap profile does:
1. Broadcast/Multicast traffic from the wired network will continue reach the RAP wired users and vice-versa.
2. The wired users are able to reach their default gateway and the controller IP on the same vlan (ARP works).
3. However, any broadcast including ARP from a wired rap user will not get forwarded to the tunnel to any other RAP wired user.
The direct impact is that wired rap users will not be able to communicate with each other, which will break the rap-wired-user to rap-wired-user phone communication.
Essentially the "no-broadcast" knob keeps wired RAP users from sending broadcasts to each other, but it does not prevent them from receiving broadcasts from the wired network at the datacenter: bcmc-optimization on the VLAN on the controller prevents downstream broadcasts from the wired network from being propagated to the wired ports of RAPs that are on that VLAN.
This is something that is not explained often, so I hope it helps.