I am not able to answer your technical questions with regards to ISO performance precisely. However, I have recently upgraded from a G2 to a G5 and find the G5 to be superior in most ways. Shadow noise at ISO 800 is much reduced from the G2. So far, I find the G5 at ISO 1600 to outperform the G2 at 800. The G5 focuses and shoots faster than the G2, and the touch interface is much improved. The new 920k dot LCD is spectacular and the EVF seems to tear less than that on the G2 (I believe it has a faster refresh rate). I feel that it has been a worthwhile and sensible upgrade to the G2. The only thing it lacks is the additional selector dial. The improved touch interface and three programable function buttons help make up for this.