The Teradek Cube will be stretched to work properly at 200 feet. I work with one all the time and there are a couple small issues with them.
The first is range. While the engineers from Teradek claim that the Cube can be stretched to around 300 feet, that was only done in a engineer controlled, line of sight test under optimal conditions. If there are other WiFi sources in the general area then the range is limited at best. When we use one, sometimes the producer can hold the iPad2 and be several rooms in a house away from the Tx unit which is attached to the camera. Sometimes not. It works fairly well passing through a few walls, but I have never seen it work effectively at anything more than 50 feet, line of sight.
The other thing some people have an issue with is that the feed isn't exactly real time. There is a true, 1/4 second delay. In the situation the OP wants to use such a device that wouldn't really matter so much, but I have seen the delay be a lot longer if the unit isn't set up properly.
And as it was stated above, they aren't that super cheap. Yes they are more cost effective, smaller and lighter than some of the other HD video transmitters out there, but $1200 or so isn't cheap for a lot of folks.
Also, the out put of the 7D isn't a true HD signal when you take it out either of the video out ports. That is really just for reference. Again, that might not matter to the OP, but something to think about.
A decend camcorder on a tripod and a long video cable might be cheaper route. But at that distance you might need a signal booster in line. Wayne might know the answer to that one.
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