Just for posterity a few tidbits of info..
The power cord on the pc-fx is attached (unlike most systems where you have to plug it in).. anyone remember the days when a lot of systems came with huge external brick power supplies to make the device itself look smaller? C='s stuff is definitely one example..
It is true that the extra 10V will mostly be converted to heat. The general advice is to keep the air ports on the pc-fx free from anything that would counteract the release of heat.. However, it has been my experience, that even with extended hours of pc-fx play, that the system doesn't even really get hot.. As a matter of fact, after 2-3 hours my dreamcast gets hotter than the pc-fx ever gets.. The voltage doesn't really seem to be a trouble in this regard.
If you only have one control pad, use the plastic bit that comes with the pc-fx (if you have it) to cover the second port. The port appears quite open to dust and I would imagine that could lead to trouble later on.
Keep all the expanion port doors shut, keep the pc-fx out of direct sunlight (like a gremlin), and make sure to keep it clean.
-RUiNa