I have an FA20-35 and the Sigma 20-40. I've tried unsuccessfully to find someone in the Phoenix area with a Pentax 20-40 who'd be willing to compare all THREE lenses.
In my personal tests, the Sigma 20-40 would get a 9 and the FA20-35 would get an 8.8, they are that close. I'd love for the Pentax to blow both of them out of the water, because then I could justify the 900 purchase. The 20-40 range is an absolute must have for my landscape work and I was thrilled a Ltd series lens was coming out... until I saw the price tag.
The review adam posted on here was less than convincing, which has further muddied the waters. And then there is the harbinger of FF. Do I even need it or want it? I don't know, but I like having a kit that is completely FF ready should pentax release one.
there's just too many conflicting opinions and variables surrounding this lens to get me to pull the trigger on a 900 purchase.
I personally say stay with the FA 20-35 until there is more evidence to justify the switch.
The more I think about it, I'd almost consider the Sigma 18-35 before getting the Pentax 20-40. Even if you went out an purchased the DA40 to cover that focal length, probably still be cheaper, and still FF capable.