Well a few things.
1. I find a lot of the ebay powersupplies are overrated. Including those supposedly intended for laptops. I would look at something more solid. The last thing you want to do is end your DSLR on a cheap $15 powersupply. It should be fine for dewheaters though, worst case scenario it burns down. Jaycar has a nice 12V 10A powersupply but it's on the pricier side. That said I'm certain you will want to power more from it soon :-) Dewstraps on camera lenses for widefields perhaps?
2. I would look at LM2596 module based adjustable regulators. There's heaps of variants available on ebay. like this one:
http://www.ebay.com.au/itm/Brand-NEW...b428565&_uhb=1
The reason I'd pick something like this is firstly you can get your 7.8V output, and secondly because they are relatively simple single chip solutions with a good reference design they are relatively hard to screw up even for cheap nasty Chinese vendors.
Now as for the design 25w is a lot to be putting through the front of the C8, though someone may correct me. I'm putting around 14w into mine. At first I found it wasn't enough but my key problem was that I simply strapped the wire on the front of my element. It lost a lot of heat to the atmosphere. It wasn't until I put a strip of rubber around it to insulate it that it worked perfectly.