To get to clean, high ISOs, you need larger and fewer pixels. To get high resolution, you need more pixels, but the trade-off will be high ISO performance. If you want a reasonable compromise, you go somewhere in the middle.
High megapixels, low noise, High ISOs – regardless of the brand of sensor, you can have two of those things, but never all three.
Unfortunately only true for today's Canon sensors.
All other manfufacturers are proving, that a good sensor design and good electronics behind it cause only minimal hits in Hi-ISO performance (mainly at ludicrously high ISOs beyond 6400) despite hi rez.
Michael Reichmann in his LL review of the A7s comes to the same conclusion:
12 MP A7s has onöly a tiny Hi-ISO advantage over 36 MP A7R ... in practive only relevant from ISO 6400 upwards.
I would be more than happy to give up 1 stop nosie advantage from 6400 upwards .. if I could get a Canon 36+ MP camera with D800E performance ... in a mirrorless cam ... sized and priced like the Sony A7R. :-)