Well, I find that taking out the color does help quite a bit. With the XL2, you can get 24p, so movement should look like film and image quality should be very good. I think using black and white video and modifying some of the colors in post (crushing the blacks, etc) would help. MiniDV loses a lot of color data when the information is put onto tape, which explains why color isn't the best hand in MiniDV's deck.
For if it can be compared to film, here's my experience. I watched Clerks a while back and wasn't impressed at all with the quality of the picture. Granted, it was an indie and shot cheap. It was shot on black and white 16mm. Honestly, it looked like it could have been shot on video.
So for a black and white feature, I think the XL2 would work fairly well. I would guess your results would get you close to that of black and white 16mm. Probably better than what Clerks looked like if you knew what you were doing. However, this is coming from someone who hasn't seen a 16mm or video movie projected onto a screen. I see my stuff through DVD. However, if DVD were your market for this, using the XL2 in black and white mode should, in my mind, give you some great results. Don't expect 35mm quality images, but you would be around 16mm / Super16mm.