I don't see any obvious problems of running RTMP on port 80.
Using RTMPT potentially and in theory will increase the latency, (see this from Adobe: http://blogs.adobe.com/connectsupport/tunneling-with-rtmp-encapsulated-in-http-rtmpt-should-be-avoided-as-it-causes-latency/). Although in my real experience the difference was not that huge.
However, what I would do is always try port 1935 first, and if it failed then try port 80 by using Cluster plugin. But again, there should be no issues in running on port 80.