On the topic of heat throttling;
Yes most of the ventilation is through the back, and it will be ventilated no matter if the lid is closed or open. The main difference is in the open scenario the chassis can heat up, and can be cooled off because the top area near the speaker and above the keyboard are opened to air. When you close the lid, the lid is now blocking any air from cooling off the top of the chassis, causing it to dissipate the heat less efficiently.
This may matter, and this may not. If your in a hot environment already, then yes it will probably have an effect. If your really making the computer work hard (particularly for a long duration) you might have problems.
In your case, using it as a front-end for plex, I don't think you'll have many problems. When sleeping you'll have no issues, it will just be with movies. If your viewing a 1080p movie, even then i don't think the GPU is working very hard to render that.
Now if your watching movies all day, and the room is hot to begin with, then I would suggest you get the iStat Pro widget for your dashboard and just bring that up and see what the temps are like.
Or if you plan to play games on it for more than an hour, do the same thing.
If you see the temps are high (over 40 Celsius for the chassis) then you'll want to find a way to open the lid, or try not to use it quite as much.
It's pretty ridiculous these claims of never running it clamshell because of heat issues, when it's a very environment-dependent situation. It will be different for everyone.
