Strange, I find that the plasma metering mode for the i1d3 is spot on compared to the i1pro2, measure's closest to the pro2 with plasma meter mode. But then again, if memeory serves me right you also have a C6?
Well, most plasmas are like yours, driven by a "600Hz" sub field, the later Panasonics use what they call a focused field drive, so instead of the image being rendered across the duration of the frame in multiple passes (10x60Hz or 12x50Hz = 600Hz), each frame is rendered in a single 1/3000th of a second burst - probably closer to the single pass nature of a CRT. On a side note, I don't know if you're familiar with the Leo Bodnar lag tester, but this behaviour of plasmas is why it can't properly measure lag on a plasma - always look at the camera method measurement, not the lag tester.
I don't have the C6, just the standard Pro model, maybe you're thinking of the HDR version of the C6 I came across in the beginning of the thread? I have no true reference measurement device, I rely on my experience as an art director and the reference ambient conditions of my room to tell me when things are off. The THX preset on my TV is pretty accurate, but a little too much red on the high end (and a little green on the low), there's a slight blue/green cast across the range when using the Plasma mode (checked to see why I selected CRT mode yesterday), CRT gives me readings than concur with my observations (both the THX preset and the plasma metering mode) and results that look right to my eye.
Typing this the thought also occurs to me that the Panasonic used a more saturated red phosphor (in order to cover 98% of DCI [and appears more natural as previously red, even if measuring correctly, could appear undersaturated/luminescent]). That might be another reason for the inaccuracy as green and blue measure the same, just wild speculation.
EDIT: It might be interesting to compare the two modes across the THX preset and my own calibration - and to compare my THX results with those of reviewers who've measured the panel with their Klein 10As. Only if you're keen to see, wouldn't just do it for fun.