Tsonga asked his question about QOS and the HDHomerun at the SiliconDust forums and got a great answer that I was looking for:http://www.silicondust.com/forum/viewtopic.php?t=5877
I never had this network throttling problem with Vista, but the same machine with Windows 7 saw pixelated recordings and live tv. Once I made the change in the registry concerning the network throttling on that network adaptor, all issues with the HDHR went away.
To answer Smarti03's question... yes you will need to setup the HDHR first before you configure VMC. Since you will be using the HDHR as a ATSC tuner, then there is very little to setup on the HDHR software's side.
As for your antenna question, you can use the same antenna for all your tuner cards/devices. But I would get a powered antenna amplifier/splitter. You can usually get a 4 port amplifier/splitter for about $20-$30 at retail stores like Best Buy or even Lowe's in their antenna/electrical department. This will save you some headache, as each time you split the signal coming from your antenna, the signal power (db) decreases a bit. The amplifier will overcome this deficiency and provide full signal power (for better or for worse) to all your tuner cards.
I say for better or for worse, because if your antenna isn't getting a good signal to begin with... the amplifier is just going to be reproducing that bad signal to each tuner.