I have the CV 15. It's a pretty good lens for the money. It's almost a given you must use the UV/IR filter on it. I put some tape on a 39 filter and it works fine, but there are other fancier, pricey fixes for this.
I use one of their 25mm lenses on my IIIf sometimes, and I like it a great deal. I was just curious if the really wide ones behaved strangely with the M8's relatively unusual sensor.
I suppose you might be able to paint the lensmount to mimic the digital encoding of the newer Leica lenses, in hopes of getting the auto processing pipeleine to reduce vigneting and the like. But it's not like Leica makes anything quite that wide anyway, so it probably wouldn't be worth it.