The plot thickens . . if I save that same raw file (using Sigma PhotoPro 5.1) as a ProPhoto 8-bit TIFF, then open it in PhotoShop Elements 6 (PSE6) there are no signs of in-camera clipping. There are no 255's and no zeros of any kind in the entire image. The lowest blue in the region of interest is 32 and the highest red is 231. The whole image histogram as shown in PSE is smooth with no tall peaks at either end that you get with clipping. I should mention here that PSE6 does actually recognize and display ProPhoto TIFF images.
So, thinking that the petal colors are probably all out-of-gamut with respect to sRGB, I opened a ProPhoto image in PSE6, reduced the saturation to move the colors into the sRGB gamut and then converted the color space to sRGB. After a few tries at that and a bit of tweaking in the final image I got this:
The highlight color clipping seen in the OP is gone. However, a lot of trouble to go to, seeing that I'm not a flower close-up shooter.
Especially when Sigma Photo Pro does the same thing in one click
("auto" button). Yep:
A bit of conversion clipping exists and the hue is a little too yellow, but not an objectionable image for one click, IMHO.
I'm back to thinking that, although ACR 5.4 may well have converted the image successfully into it's RIMM 16-bit working space, the image's journey from there to a) ACR's histogram, b) ACR's color-picker and c) onwards to PSE6's ROMM working space was less than smooth. What do y'all reckon?
Comments are still welcome.