I'm back, after a some more research, some experimenting with the sunflower and re-thinking the reasons behind the OP.
For me, my sunflower problem posted earlier occurred with ACR 5.4's conversion from X3F into it's working space. In that version of ACR what you see (all sliders 0) is what you got, assuming WB to be as good as it can be. That is to say, the conversion itself is fixed by Adobe. If there is a clipping problem caused by an image occupying a large gamut, that can not be fixed in ACR. That is to say, the image appears on the monitor clipped or blotched and, only when the sliders are set to extreme levels downward do the blotches go away - at which point the image looks stupid, washed out, a total failure. It may well be that the in-camera exposure was high for the shot (it was a one-off snap) but the RAW data itself was not clipped - I checked and re-checked.
On the other hand, Dave Coffin's 'dcraw' allows all sorts of adjustments to the raw data during the conversion
, among them being saturation, brightness and gamma - not to mention a choice of output color spaces: raw, sRGB, aRGB, aWIDE, PP and XYZ. Not that I recommend dcraw particularly, it has a clunky command line interface and a somewhat protracted work cycle.
So it is that, armed with a better understanding of color spaces and profiles gained from this thread, I've managed to produce a blotch-free sunflower (again) but this time with some knowledge of how that was possible.
(Slightly sharpened, no other processing).
I no longer regard 'perceptual' as a kind of Grail, especially for one who only produces sRGB images and never prints anything. I will continue with what applications I have for now. Any 'difficult' capture will get either dcraw'ed or trashed; ACR->PSE for the rest . . ;-)
Perhaps I should mention that my serious cameras are RAW only (early Sigmas).