"1. Venus does *not* receive 2x Earth insolation at the surface. It has an albedo of over 90%, if I recall, vs about 35% for Earth (all those clouds). Additional energy probably goes into heating the lower cloud deck vs the surface. Total energy received there is pretty low compared to Earth in full sunshine."

At the surface of Venus it is pretty dim, but it much brighter than if you were in 90+ atm on Earth [under 2700' of water].

Venus is a world with a global "ocean" of CO2. The CO2 gas is fairly transparent. If you were on Venus at altitude in which it was same pressure as on Earth- the sun's intensity would be more than earth- not sure it would twice the energy though. CO2 does block more solar radiation than our atmosphere. But I would guess that solar panels would probably produce about twice the energy as on Earth.

"3. Furthermore, CO2 is only efficient at absorbing a limited slice of IR spectrum, and as you increase the temp you also increase the peak wavelength being emitted. CO2 has its peak absorbption at 4 and 15 microns. At 300 K Earth's peak emission is at something like 12 microns, and Venus at around 3 or 4 microns. As Venus gets hotter, a larger and larger share of its emissions aren't blocked."

This might be right, but what if you apply this to Mars- Mars is suppose to have greenhouse affect which adds 5 C.
"Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K)"
http://www.nineplanets.org/mars.html
At least during the day on Mars the surface would warmed around 300K.
In addition Sunlight on Venus's surface is pretty feeble- what warming the surface would not be direct sunlight but rather it would heated from the atmosphere.

"So you're right, you can't just say that total mass of CO2 linearly translates to temperature, certainly not when comparing concentrations different by a factor of 10,000. Does that mean 100ppm to 200ppm has less than a x2 increase, probably. But zero or negligible? That doesn't follow at all."

Well, I hope wasn't suggesting zero or negligible- just that it HAD to be less. I wasn't saying how much less, but 5% or 10% less was more in line with what I thinking about rather than 95% or 100%.

">It can only mean that most of this warming is warming nites and winter areas. What it does NOT mean is that most warming is added to daytime and particularly to warmest period of the day temperature.

Please perform the following experiment. On a warm sunny day, lock yourself in your car and roll up the windows. Please report back to us on the results. "

This would prove nothing. Whole idea of a greenhouse was to prevent freezing during the nite- it allows someone in England to grow tropical plants. It also allows one to have extending growwing season in cooler climate.
But greenhouses or cars with windows rolled up are more about blocking heat loss due to convection of the air.
Better experiment- go swimming at midnite.