neilm16 wrote:
...has it always been the case that the sensor used in earlier models like the G2 and GF2 has matched DSLR IQ upto say 800 ISO...
On one hand, no, because at ISO400 and below, there is very little difference between the G3/GX1 sensor and the previous gen G2/GF2 sensor. It's only when you get to ISO800 and above that the new sensor improves over the old and "closes the gap" so to speak between itself and the sensor in the D5100.
On the other hand, yes, because at ISO400 and below, the gap between either generation of Pany sensor and the Nikon sensor is small enough that the skill of the photographer and post-processor will have a more significant impact on the resulting images than the sensor tech.
Mark