Several points.
When an AP is doing 300Mbps, that's NOT the real throughput you have. It's the data rate at which traffic is sent.
All in all, if your AP/client are doing 300MBps association, you will see max 150Mbps with a file transfer.
From there, I'm not even sure that 11n supports dual spatial streams over such long distances (you can't have multipath in open air) so afaik the 1550 only do 150Mbps association rate (=dual channel with one spatial stream). That means 75Mbps real speed.
I couldn't test a 1550 yet so don't take my word for official statement but that's what I'm thinking.
the wireless links will always be both up and they can be on different channels.
That will then mean that it will be "as if" the remote switch was connected directly to the central switch (where WLC is connected) as the WLC tunnels traffic all the way. So you could do a spanning-tree config on this one I guess to block the port onthe remote switch.
Regards,
Nicolas