The Colour space that is set in the camera does not impact on the final file as it would in the JPEG file, as colour spaces are assigned in the raw conversion. To be honest though there would be no point to using the Prophoto colour space as there is no way that you can actually see most of the colours in that colour space.
One thing to bear in mind though is your file names, when the camera is set to Adobe98 the file names start with _XXX0001.DNG while with sRGB the files are XXXX0001.DNG, something to consider.
A weather sealed ultra wide, is that too much to ask?
Pentax SLR talk FAQ