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We're talking about different pollutants here. If you allow your cat to get too cold, you're going to see a sharp rise in NOx and HC emissions as the cat doesn't function when it's cold. However, automakers were able to squeeze engine stop/start systems past the EPA, and some were able to achieve a SULEV rating while doing so. Whether this is an artifact of the EPA's testing methodology not representative of the actual NOx and HC emissions of hybrids, I'm not sure. It's at least an indication that engine-off is inoffensive enough that the EPA condones it.
Some cars run rich after a key start, others run rich even after a bump start. Some do not. Running rich creates lots of HC pollution, and CO2 as well.
The one pollutant all of us can measure is CO2. The ScanGauge will give you a pretty close estimate of how much you're emitting. P&G has been shown to reduce your CO2 emissions substantially.
Factoid: You can reduce your NOx emissions by cruising down the highway in 4th gear instead of 5th.
Reality: The resulting increase in CO2 emissions have* a greater GWP than the NOx you'd emit, even in lean burn.
*I haven't done the math, but if you have a three-gas analyzer and an mpg gauge, you can. I did do the math for the SULEV CVT Insight without lean burn versus the LEV MT Insight with LB, and the MT has the lower GWP despite higher NOx emissions.
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