Does any company make Back-side illuminated sensors in Aps-C or FF Size? Wouldn't this tech allow a massive jump in ISO performance?
No. It doesn't make sense from a cost-benefit viewpoint.
The reason that rear illuminated sensors are good for tiny sensors is that each pixel that needs to be read requires a certain amount of space for the 'wire' to read the signal from that pixel. The space taken up by the wires used to read the pixel cannot be used to capture light.
(Almost) The same amount of space is required per pixel regardless of what the sensor size is.
So on a tiny little 1/2.3" sensor changing to rear illuminated means that there could be 50% space for capturing photons. On a 1.6 crop or full frame sensors, the space taken up by the 'wires' is really small compared to the area used to capture light, so you could maybe get 5% more space for capturing photons by switching to a rear-illuminated sensor.
Because they're harder to make (which translates to more expensive to manufacture and higher defect rate) there's no point getting a 5% increase in light capturing on a large sensor, whereas getting 50% more light in tiny sensors does make sense.