After the Canon 30D came out, I inspected some of its RAW files, and found that the new 1/3-stop ISOs were nothing more than implicit EC-adjustments to the old 100/200/400/800/1600 20D series. To make a log story short, ISO 125 has as much shadow noise as ISO 640, the extra ISOs all lose 1/3 stop of highlights; half of them lose it completely and the other half shifts it to the shadows. The results are spiked and gapped histograms.
Someone asked me about the 5D after I posted my conclusions in DPR. They linked to some 5D files that I inspected, where I found the histograms to be unaltered for the in-between ISOs. The thread got revived again, and the 5D came up again, and someone linked to some blackframes from the 5D at all ISOs. Histograms looked the same (although *all* had a tiny stretch with gaps about every 15.5 RAW values), but the blackframe noise, unfortunately, had some very bad news ... 160 is higher than 200, 320 is higher than 400, etc. The 100/200/... series was the best; the 160/320/... group were the worst. My conclusion at this time is that the 5d uses analog amplification for the extra ISOs, but it is futher amplifying a subset of the ISO amplifications, to get the rest.
My question is, do any of the cameras out there with "in-between" ISOs that actually have "in-between" noise, or are they all trying to deliver technology that they don't really have?