The current thinking in the dimmer world is that when using Mains Halogen lamps, dimmers should be de-rated to half their stated maximum (plus a bit). In other words a 400W dimmer should be fine for 250W Mains Halogen.
For Low Voltage, everything depends on the quality of the transformer...
Just found this site and I have some connection with Varilight so I may be able to help.
This problem that you report has been noted before in a very few cases. A common cause of lights dimming themselves when using Touch / Remote dimmers is that an unsleeved earth touches the heat sink on...