Have you compared ISO 1600, 2500 and 3200? Canon only has 'real' ISOs on the full stops, everything in between is just brightened up a bit by software. ETTR on ISO 3200 should look a lot better than plain ISO 2500. Where 'a lot' might just be placebo effect.
In my experience, the "-1/3 stop" ISO settings (160, 320, 640, 1250, 2500, etc.) that are "pulled" 1/3 stop in raw development (so "darkened" by 1/3 stop, rather than "brightened" by 2/3 stop) are about the same as the full stop amplifications at which they are actually shot (200, 400, 800, 1600, 3200, etc.) with a 1/3 stop loss in highlight headroom. It's the "+1/3 stop" settings (125, 250, 500, 1000, 2000, etc.) which are pushed 1/3 stop in development from the full stop amplification at which they are actually shot (100, 200, 400, 800, 1600, etc.) that are noisier than the full stop settings well above them.
A few old tests done when this first burst into public consciousness a decade ago support my experience. Here's one of the easiest to digest at a glance of the graph. A lot of the links I have to such similar tests are now dead.
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