Multicast on PTMP wireless is treated as Broadcast. Think about it, there is really just one path to all clients -- unlike a switch with multiple paths to clients. This also means that the multicast/broadcast goes out at the slowest Basic rate of the 802.11 device not the fastest because the AP must ensure that all clients hear the broadcast/multicast packets and so it must employ a least-common-rate logic. So one client which runs at 1mbps will spoil all the 48mbps clients, so to speak.
The exceptions here are:
1) You force all clients to a minimum Basic rate, thus the slowest Basic is at, or close to the fastest. Since this means you are forcing all clients to the lowest possible rate of the worst client (signal wise) you are de-optimizing the wireless network. This is only feasable if all the clients are very strong signaled.
2) You encapsulate the multicast traffic in a unicast tunnel. Of course, this means that a multicast to 5 clients must be sent via 5 unicast packets, so this solution only works if you have one, or maybe two, clients who need to pass multicast.
So the simple answer is that 5 customers watching a 1mbps multicast steam will cause the AP to send it only once, but at the slowest Basic rate of ALL the clients. This means you use up the worst-case airtime. I assume this completely screws your business model.