The Benq w6000's IRIS can be adjusted manually down to produce almost the same brightness as a Mits hc4000. Both projectors can get under 500 lumens, so the Benq w6000 isn't really an issue even for smaller screens either (although you will lose the DI ability while the IRIS is closed).
The Mits hc4000 produces a better image than the Optoma hd33 or Epson 8350, but of course it has no 3D. You will sacrifice some 2D quality going to the Optoma hd33, but according to some that have owned both, it's not the end of the world and it's still nice to have 3D I suppose.
The IRIS on the Benq is somewhat aggressive in order to achieve those darker blacks, so it is not a pure even trade. The hc4000 actually has a higher native contrast ratio than the Benq w6000, but the Benq's IRIS just closes down so far that it gives it the dark blacks. For people that do not like IRIS movement, the Benq may not actually be the best projector. There are advantages / disadvantages when comparing the Benq w6000 to the Mits hc4000. The Mits hc4000 has a cleaner more noise-free image, but the Benq has the IRIS and is brighter, and a bit sharper (which doesn't matter much at these levels). The Mits hc4000 is the only Dark Chip 3 projector I know of that is under $2000, even the Benq w6000 is only Dark Chip 2.
The Benq w6000 doesn't blow away the picture of the Mits for how most would consider it, so if you want to blow the picture of the Mits away, look to an Infocus sp8602, which has an IRIS and better native contrast. There is also the Optoma hd8300, older hd8600, newer Mits hc7800, and they will all also easily cream the Mits hc4000 in PQ.
One of the reasons the Benq w6000 has fallen in price so much is because it is just not as far ahead of some of these newer projectors as it used to be against older projectors, but the w6000 still has the placement flexibility which is nice. Whether or not you think the Benq will blow away a lower-end DLP is really going to depend on how much you like the IRIS vs. how much you'd rather have pure native contrast, as well as how much image noise bothers you. The Benq also has the noisier fan and added noise due to the IRIS.