Stef, I can guarantee you that my P21 outranks the A900 (I have the A850 to which I can compare). The sheer dynamic range of 12 stops is something no DSRL camera can even dream off.
Working with equipment as such however requires a total different approach, you are not exactly going to shoot 'street' with a set-up like that.
I use my SWC with P21 for landscapes and city shots and each shot requires a lot of preparation, always shooting from tripod. DB's are indeed very popular in the studio but that is not my kind of shooting.
The Phase One + series these days have very good high ISO performance going up easily to 3200 or more.
Nevertheless, don't get me wrong, I understand your concern. I have the A850 combo with the 16-35/2.8 and the set-up is a whole different world alltogether.