Running more than one output is not advisable on a Mac Mini, let alone DH2G or TH2G.

Using a 2009 server version with SSD installed and Snow Leopard, and everything streamlined, I have managed one output at 1920x1080 running a single layer of 1920x1080@60Hz AIC.

Have also run and also 2x 1024x768 running NTSC content. (2 layers)

Much more than this, content depending - can be problematic.

If you want to do any more - I'd really recommend a MacPro.
If your budget doesn't stretch to a new one - look at Apple refurbs for the 4,1 (2009) version.
Alternatively - there are some suppliers with old stock of 2009 quad core MacPros still available, which are a good deal.
You can even get good deals on ebay...

As for performance - there are many variables that affect performance - so you must try to test your setup/scenario yourself so see if it is feasible. Can you borrow a Mac to try it out? - either way - invest in an SSD for content playback. (OCZ Vertex are a good deal)

Let me know if you need more help

Simon