[sparkytwd]

From those speeds it does sound like you're hitting USB bus limit. Not sure what the topology of U3's ports are, but on the XU, there were a few distinct end points, which means you can potentially have separate bandwidth for multiple cameras.

If you want to play around with cameras and compressed formats, give the C920 a shot. It supports both MJPEG and H264 compression. You can use gstreamer and v4l2loopback to convert the video to whatever format you'd like.