Question: why use a rms calculator chip? I would rather use two adc channels and sample both I and U at the same time, and calculate the result. This way you can also measure power.
Also note that you don't need absolute add precision for your 6000 counts, you can use 12 bits, for example, the massive oversampling gives you the needed precision.
Concerning chips out site specs, it can be OK, if the risks have been properly rated, and the outside spec validated as thoroughly as possible. A eol test is a good bonus. I seen it done at a big Corp for million automotive products. It was done properly.