I currently run a MBL with the most recent firmware. I have installed Twonky 8 and many other programs (Transmission, OpenVPN, ownCloud, and many others). My MBL is not overloaded at all. In fact it runs at low CPU usage rates. And i have no problems with sluggishness when streaming to two or three devices wirelessly from the MBL Twonky media server simultaneously. That said, I also have a cheap 80mm fan under the MBL keeping it cool. I found that the biggest problem with MBL performance was that the CPU got hot and then became sluggish.
There is an excellent hack called FeaturesPackManager that you should install. It will allow you to see the performance stats of your MBL so that you can determine if you have a MBL sluggish problem or if it is actually just a problem with your device receiving the media stream.
