I have a P30+ and I would have to disagree.
My opinion is entirely subjective, but I will say that you get better color from an F5 and 35mm Portra 160nc than from the 1ds3. I would rather (and do) shoot editorial on film (645 afd, 6x7 RZ) than shoot with the ds3. The P30+, again, in my opinion, has qualities that are unique to medium format (lenses, 4:3) and the dof effects from a larger sensor are real, so there is, how you say, the MF "look". So unless the 1ds4 has a larger than 35mm frame, is 4:3, and takes mamiya and Hass V glass, I don't think Canon will ever catch up to the P30, A17 or P21.
As to iso, the P30+ is very similar to my 1ds2 at 1600. More color noise, cleans up well in C1 or in noise ninja. The pixel density masks much of the noise. At 400 there is virtually no noise to speak of, its like an A22 at 100.
As to fast lenses, its all relative. Fast for thin dof? MF, even the P30, beats 35mm with 2.8, 1,9, and F2. Fast for low light street shooting? Well, mfdb are really not the best choice. All that being said, if I shot sports or street or something like that I'd shoot a 5d or 1ds3 or D3. But I don't. This is not to take away from the 1ds3, its a great cam but it will never be a MFDB system beater for my work.
I find all of these cameras, dslrs, medium format to be very scene and lighting specific.
Testing with lots of front and side light just about anything looks good, even at high iso, but then again it's doubtful anyone would intentionally shoot 800, or 1600 iso with studio strobe and a lot of fill.
Where I see at difference is in the times you really need high iso, dimly lit NY streets with soft fill, backlight on location with hmi and natural fill, wider apetures, slower shutter speeds, then the noise comes in.
In those instances even my 1ds2 shows a lot of noise in the shadows and not very pretty noise at that. The D3, well at 1000 iso is pretty clean, though the color is sensitive and under tungsten I find all the medium format backs superior to the Canons, (though I haven't tried a 1ds3).
Even my past Aptus 22 was really nice at 400 iso, as long as I wanted the noise to be visable, though unlike the Canon it was pretty noise, almost random and not clumps.
For commercial work, especially commercial fashion where there is a lot of backlight and wider apertures, the P30+ is good at 400, slightly challenged at 800 and I know nothing about 1600 as i've never tried it.
Keep in mind I said commercial work, where I know someone is going to be looking hard at the detail of a dress, or jeans.
Also, all of these cameras show different results depending on the raw processor.
Right now I like the look of C1 v4, for the p30+ and my p21+.
Once again, all of this is subjective, but preparing for a shoot this week, I tested the P21+ (I hate writing that plus thing) vs. the p30+ at 800 iso and though 800 is supposedly the top iso for the p21, at 800 iso and underexposed slightly more than 1/2 a stop it cleaned up well as long as the image compression was set to large rather than small.
On small compression V4 handled the noise very abrutly, either too smooth and paintrly, or way too noisy. One large compression v4 worked the p21 file like it was an entirely different camera.
Pusing it +.64 in v4 which I assume gives an effective 800 iso something closer to 1000, it was really nice, not smooth plastic clean, but more random film clean and actually something I would do even for commercial work.
I gotta stress, the difference at high iso between large vs. small compression is very noticeable.
Still, this is subjective, but I like the p21 look at high iso because it's almost film like grain to me and not just deep chroma noise.
Then again I've always shot the phase backs slightly under and like the results better than the expose to the right scenario we read about.
This is not an offical company line, but a year ago one of the Phase people asked me how I exposed the file and I said slightly under and then opened it up in the processor and he replied yes, that is the way to do it.
That is not an offical statement and there could have been something lost in translation, but it works for me.