My thought would be stick with something stiff for those skis, especially if you are going to be doing dh runs mostly.
First off - what are you wearing in them? My wife wears her plastic boots even in 0°F days with just an ultra-thin sock. You could probably get away with just a bike sock, or a special ski sock (basically a sock liner).
You could go leather but I think if you were to order the same size Scott Excursion you'd find a more comfortable, and flexible fit.
Kind of expensive, but if you're dying for boots NOW, this is an option:
http://nextadventure.net/scott-excursio ... oCl-bw_wcB
The other option is get a thermo-moldable liner and go see a boot fitter. They'll always tell you use a tight boot and they can stretch it out in the tight spots. As long as it's not too short overall they can fix it... but it will cost you. I'd be prepared to spend another $300+ for a liner and labor. If you can find a boot that fits better out of the box, it will be cheaper in the long run.
From what I'm reading from other member the BCX675 might be a little weak for those skis. Seems like there are some flaws with that boot that will probably not be great with a wide ski like that.
You might want to go a little forward of chord for those skis, maybe 1-2cm. Is there a boot center mark? For some reason I though Rossi's had boot centers marked on them, but maybe that was not the BC series.
Anyway pin line 2cm forward of chord is where a comparable ski like the Annum would recommend mounting for your boot size. It will make a difference with pins and soft boots. You can't weight the front of the ski like you can in Alpine, so mounting forward will help them turn. Balance is useless IMO for a ski like that. A nicely designed ski, like the Fischer S Bounds, has the balance a cm forward of chord, but I don't know that it matters much... you're not skiing in a track.