Thanks for the response, OzJohn.
I was a little unclear about what I meant by the E6/C41 equivalence. What I mean is making the characteristics (color, contrast, resolution) of the print similar to those of an E6 transparency. I appreciate that one is a reflective medium and one is transmissive, but I wanted the print to look similar to the Velvia on a light table. To that end, I have been loading two backs with E6 and C41 film and making identical exposures with each (using Ektar and Velvia, even the speed is the same). I take mostly static subjects always using a tripod.
Using Ektar (store processed), the density range of the negative is definitely greater than that which can be handled by the print medium (Fuji CA). I can increase the aperture of the enlarger lens and get the highlights in range but not the shadows and reduce the aperture and obtain the reverse situation, but never both at the same time with Ektar.
I am not too bothered by the consistency or color of the prints. In fact, I am amazed by the robustness of the RA-4 process -- I have changed various processing times by factors of two with almost no visible change in the result. The changes in the print appearance were almost undetectable when I dumped my six month old chemicals and used fresh stuff.
After a few trials (and with the help of the Kodak viewing filter kit), I can get reasonably accurate color. I therefore don't think that the temperature is the problem, unless it affects the contrast also. By the way, numerous posters on APUG seem to have a similar experience as I have, reporting that the chemicals can be successfully used at room temperature for many more prints per liter of solution than Kodak recommends.
I do have a water bath with a very precise temperature controller for my E6 work, but it would be difficult to use for tray processing. (I have ordered some drums, mainly to reduce my exposure to the fumes -- I might try processing in the drum at 38C when I get them). I also find that my E6 processing seems to not be bothered by using older concentrates -- I just processed my fifth roll of E6 using chemicals (Kodak) which were first opened almost a year ago and the results were perfect, so far as I can tell and I examined the slides under a microscope).