We have been using the DELL WYSE P25 units (Tera2) for about two months now and have found no real desktop performance enhancements over the Leadtek VP200P units (Tera1).
The P25 units run a lot cooler and the USB 2 devices performance is much better.
For real enhancement in desktop performance I would recommend you look at installing a GPU in your host e.g NVidia Quadro 4000 we are so far really impressed with the desktop feel and performance.
Maybe have a look at the NVidia Grid cards http://www.nvidia.com/object/grid-boards.html
DELL WYSE P25 http://www.wyse.com/products/cloud-clients/zero-clients/P25
Cheers.